With thousands expected to protest TransCanada's proposed Keystone XL pipeline at a White House rally on Sunday, President Obama said it could be “several months” before he decides whether to OK the project and pledged that he would not put safe drinking water, fertile croplands, and public health at risk.

“My general attitude is, what is best for the American people? What’s best for our economy both short term and long term, but also what is best for the health of the American people,” he said Tuesday in an interview with KETV in Omaha, Nebraska.

“I think folks in Nebraska, like all across the country, aren’t going to say to themselves, ‘We’ll take a few thousand jobs if it means that our kids are potentially drinking water that would damage their health,’” Obama said, “or, if rich land that is so important to agriculture in Nebraska ends up being adversely affected, because those create jobs and when somebody gets sick that’s a cost that the society has to bear as well.”

The comments, Obama’s most comprehensive to date about the Keystone XL project, highlighted the sensitive political balance the president is trying to strike. The issue has become a political hot potato that threatens his relations with a core contingent of his progressive backers.

The whole affair may reach a tipping point this week, when Robert Kennedy Jr., environmental activist Bill McKibben, and thousands of other pipeline opponents are expected to join a Sunday rally at the White House in what could be the largest pro-environment demonstration of Obama’s presidency.

Just a few months back, few Americans had heard of Keystone XL, which would carry Canadian tar sands crude to the U.S. Gulf Coast. Now, the New York Times has expressed opposition. Senate Majority Leader Harry Reid, D-Nev., wrote a letter to Secretary Clinton in early October, assailing the “dirty and polluting oil” that would come from the tar sands and urging “job-creating clean energy projects instead.” And while delivering a speech last week at the University of Colorado in Denver, President Obama was called out on the Keystone XL pipeline by native American leader Tom Poor Bear, who later posted his comments here.

“We’re looking at it right now,” Obama replied, as Poor Bear was ushered out of the venue for disturbing the president’s speech. “No decision’s been made,” Obama said, “and I know your deep concern about it, so we will address it.”

This Sunday’s rally in Washington builds on August protests during which more than 1,200 citizens were arrested in front of the White House. Among the keynote speakers Sunday will be NRDC founder John Adams -- who last visited the White House in February to receive his Presidential Medal of Freedom for four decades of environmental advocacy -- and activist Bill McKibben, one of the organizers of the event.

Actor Robert Redford has cut a video urging people to attend, as has David Strathairn. And Mark Ruffalo has addressed protestors at Occupy Wall Street, urging followers of that movement to show up to demonstrate against Keystone XL on November 6. Protesters are expected from Nebraska, where ranchers, farmers, and others worry that the pipeline could threaten critical underground water sources. That’s why Governor Dave Heineman called the special session of the state legislature that started Tuesday and  is expected to last at least two weeks.

At issue is whether the 49 senators in Nebraska’s unicameral legislature will pass a law requiring the pipeline to be moved. A pair of legal memos lay out the authority the state might invoke. Live coverage of the legislature is provided here.

What’s got Nebraskans on edge is that the pipeline would cut through 247 miles of what's called the Northern High Plains Aquifer System, according to the final environmental impact statement the State Department published in August. Made up of the Ogallala Aquifer and four smaller underground water sources, this system provides 78 percent of the public water supply and 83 percent of the irrigation water in Nebraska. The state relies, absolutely, on a clean underground water supply.

In the Sandhills and near the Platte River Valley, 65 miles of the proposed pipeline route would cut across groundwater supplies just 10 feet, or less, below the surface. A worst-case spill from a pipeline accident could contaminate as much as 4.9 billion gallons of water in that aquifer, according to a study by John Stansbury, PhD, of the University of Nebraska. You can read the report here.

Here’s some background on the Keystone XL project that coiuld be helpful to anyone seeking to understand how we reached this point and what's at stake:

By the Numbers

In September 2008, the Canadian company TransCanada Keystone Pipeline, LP, applied for U.S. permission to build and operate a 1,700-mile pipeline to carry tar sands crude oil from the province of Alberta to depots in Oklahoma and Texas, where it could be exported or refined into fuel.

The $7 billion pipeline would carry up to 830,000 barrels -- or 35 million gallons -- a day of diluted bitumen, a low-grade crude oil extracted from tar sands in Canada's boreal forest, one of the largest contiguous wild forests in the world. The pipeline would snake through parts of Montana, South Dakota, Nebraska, Kansas, Oklahoma, and Texas, slicing through the Great Plains and crossing 1,590 public and private roads.

Construction would tear up about 24,134 acres of land, 15,341 of which would be restored. Most of the rest, including about 300 acres of wetlands, would become permanent pipeline right-of-way, with the pipe itself buried several feet below ground, according to the State Department.

The pipeline would cross 1,455 American waterways, from the Yellowstone River in Montana to Pine Island Bayou in Texas, in most cases crossing beneath the creek, stream, river, or lake. And it would pass within 1 mile of 1,850 wells, ranging in depth from surface-level to several hundred feet, that provide fresh irrigation and drinking water for farms, ranches, communities and homes.

Opponents of the pipeline fear it would put the country’s heartland at risk of the kind of accident that gushed 42,000 gallons of toxic crude oil into the Yellowstone River just last summer. And, they point out, Michigan's Kalamazoo River has yet to recover from the pipeline rupture that spilled 840,000 gallons of tar sands crude last year, just one of 36 major pipeline accidents that killed 22 people in 2010 alone, the U.S. government reports.

The National Interest

Under the terms of Executive Order 13337, signed by President George W. Bush on April 30, 2004, TransCanada's application is being handled by the Secretary of State, in consultation with the Environmental Protection Agency, the Department of Transportation's Pipeline and Hazardous Materials Safety Administration, the Department of the Interior, and several other federal agencies.

If Secretary of State Hillary Clinton determines that the pipeline would, in the words of the executive order, "serve the national interest," she may propose that the project be approved. If not, she may propose denial.

The determination of national interest involves several considerations, including energy security, environmental, cultural and economic impact, and U.S. foreign policy. Secretary Clinton’s decision stands unless any of the consulting agencies disagrees. In that event, the president makes the final call as to whether the project is, or is not, in the national interest (which, as indicated above, he has said he will do).

The administration has indicated it will make its decision before the year is out. Last week, however, State Department officials told reporters, on background, that the deadline might slip beyond year's end.

The Dirtiest Oil on the Planet?

Tar sands crude is strip-mined or drilled from Canada’s Great Boreal Forest, one of the last truly wild places on earth, using one of the most destructive industrial practices ever devised. To date, the process has turned a 232-square-mile area -- about the size of Chicago -- into a ruin of open mines, toxic sludge, and waste ponds that threaten area ground water, rivers, lakes, and streams. (See "Canada's Highway to Hell," Fall 2007.)

Producing Canadian tar sands crude and turning it into fuel produces more of the greenhouse gas emissions that are warming the planet than any other major crude oil source, according to a March 20, 2009, report by the U.S. Energy Department's National Energy Technology Laboratory, titled "Consideration of Crude Oil Source in Evaluating Transportation Fuel GHG Emissions."

Getting tar sands fuel from the ground to the fuel tank generates two-and-a-half times the greenhouse gas emissions of domestic U.S. crude and nearly twice the emissions of the average crude used in this country, domestic and imported. For that matter, Canadian tar sands crude is almost twice as carbon-intensive as conventional Canadian crude, according to the report.

The production of tar sands crude relies on large volumes of hot water or steam, typically heated by natural gas. Producing bitumen crude from Canadian tar sands generates about three times the greenhouse gas emissions generated to produce the average crude oil used in the United States and four-and-a-half times the emissions generated by producing domestic U.S. crude.

The Royal Society of Canada has warned that tar sands production could derail Canada's goals for reducing its national carbon footprint. In a recent report (p. 292), the society writes that tar sands emissions "pose a major and growing challenge to Canada's ability to meet national GHG (greenhouse gases) emission reduction targets in keeping with international GHG reduction targets."

Tar sands crude requires extensive refining. Turning Canadian tar sands crude into diesel fuel, for example, generates 71 percent more greenhouse gas emissions than are given off turning domestic U.S. crude into diesel.

In terms of producing crude oil, only Nigerian crude is dirtier than Canadian tar sands. The tar sands crude, though, is heavier and has five times the sulfur content of Nigerian crude, requiring more extensive refining. When the additional emissions from that is taken into account, the Canadian tar sands crude generates more pollution overall.

The State Department calculated that replacing the current mix of U.S. and imported crude oil with bitumen from the Keystone XL project would add up to 21 million tons of carbon dioxide to the atmosphere annually, equivalent to the emissions from 4.1 million cars.

For these reasons and others, environmental groups commonly refer to tar sands crude as the dirtiest fuel on the planet.

Environmental Impact = Political Controversy

On August 26 of this year, the State Department issued the final environmental impact statement for the proposed pipeline, concluding that the project would create “no significant impacts to most resources along the proposed Project corridor,” provided the pipeline is built and operated as stipulated in the company’s plans and the impact statement.

The State Department prepared the impact statement with help from a Houston-based consulting firm, Cardno ENTRIX. That company, though, has financial ties to TransCanada, the New York Times reported, saying that, by assigning the study to Cardno ENTRIX, the State Department was “flouting the intent of a federal law meant to ensure an impartial environmental analysis of major projects.”

There have also been charges that some State Department officials might have helped to coach TransCanada through the permitting process, in a way that could compromise the neutrality of the U.S. government in the process. And some in Congress have raised concerns that TransCanada may have benefitted from special access to Secretary Clinton.

Last week, Sen. Bernard Sanders, I-Vt., Sen. Ron Wyden, D-Ore., Sen. Sheldon Whitehouse, D-R.I., joined Rep. Peter Welch, D-Vt., and seven other members of Congress in sending a letter to State Department Inspector General Harold Geisel, seeking “a thorough investigation” into whether State Department officials acted inappropriately or illegally throughout the process. The group sent a second letter to President Obama expressing their “many serious concerns.” You can read both letters here.

Last week, NRDC, the Sierra Club, and ten other environmental groups sent a letter to State Department Inspector General Harold Geisel calling for such an investigation.

The EPA was critical of the impact statment when it came out in August. Last week, EPA Administrator Lisa Jackson said the agency is poised to release additional comments as early as this week.

NRDC and other environmental groups have also objected to the impact statement, claiming that it inadequately reviewed wildlife impacts, safety issues involving the transport of bitumen, alternate pipeline routes, and refinery pollution, among other issues. Another party to the letter, Friends of the Earth, has pulled together a useful website on questions raised by the State Department’s handling of the matter, calling the department’s review of the project “deeply, irreparably flawed.”

A Jobs Dispute

TransCanada claims the pipeline could generate 20,000 direct jobs. The State Department found that number to be inflated more than three-fold.

"The construction work force would consist of approximately 5,000 to 6,000 workers, including Keystone employees, contractor employees, and construction and environmental inspection staff,” reads the final environmental impact statement the State Department published in August of this year.

And the Cornell Global Labor Institute issued a report concluding that the project might kill more jobs than it creates by dampening investments in renewable energy and efficiency gains that are creating American jobs.

See OnEarth's complete coverage of Keystone XL.