What's the big deal about the Keystone XL tar sands oil pipeline?

Over at Mother Jones, Kate Sheppard has a good summary of the issues. We've been giving ongoing coverage to the two-week long protest at the White House, as activists try to pressure President Obama into doing the right thing and denying approval for the Keystone XL pipeline.

• Keystone XL  is a 1,661-mile pipelinethat would carry oil from Canada's tar sandsto refineries in Texas.
• If TransCanada gets the green light from the Obama administration, the pipeline would carry as much as 900,000 barrels of oil every day—oil with a carbon output 20 percent higher than conventional oil supplies.
• The existing Keystone line (which XL would extend) has already leaked a dozen times in just one year of operation. The Keystone XL would cross more than 70 rivers and streams, including the Missouri, Platte, Yellowstone, and Arkansas.
• It would also cross the Ogallala Aquifer, which provides nearly one-third of the groundwater used to irrigate US crops, supports $20 billion in agriculture, and supplies drinking water to about 2 million people. A worst-case-scenario spill in Nebraska's sand hills above the Ogallala Aquifer could dump as much as 180,000 barrels, tainting the vast water supply in the region.
• Who is opposed to building the pipeline? Environmental groups, landowners along the path of the pipeline (especially those threatened with eminent domain), the National Farmers Union, climate scientists, a number of senators (including both the Republican and Democratic senators from Nebraska), the Transport Workers Union, and the Amalgamated Transit Union have all urged the State Department to veto the plan.
• Who supports building it? TransCanada, of course, as well as the oil companies that plan to ship oil through it, the American Petroleum Institute, the Teamsters Union, the US Chamber of Commerce, and the members of the House that voted for a bill that would expedite consideration of the plan.