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does not miss another construction season, the senators' letter said.
• But politics likely is a larger factor. Six of the Democrats who signed the letter face challenges this year: Mary Landrieu of Louisiana, Mark Begich of Alaska, Mark Pryor of Arkansas, John Walsh of Montana, Kay Hagan of North Carolina and Mark Warner of Virginia.
• Democratic efforts to keep control of the Senate could hinge on those races. All but North Carolina are significant energy-producing states. Obama lost all but Virginia in 2012.
• The Keystone XL pipeline has emerged as an election-year dilemma for Democrats.
• Wealthy party donors are funding candidates who oppose the project -- a high-profile symbol of the political debate over climate change. But some of the party's most vulnerable incumbents are pipeline boosters, including the six who signed the letter Thursday.
• The Republican-controlled House has voted several times to approve the pipeline, which has support from a majority of senators. Senate Majority Leader Harry Reid, D-Nev., blocked a vote last week on a Republican proposal that would have allowed construction of the pipeline and made numerous changes in the nation's health care law. GOP lawmakers say all of the proposals would help create jobs.
• Polls show most Americans support the pipeline. Sixty-five percent of those polled backed the project in a recent Washington Post-ABC News poll, while 22 percent opposed it.
• Several former Obama administration officials, including ex-Interior Secretary Ken Salazar and former national security adviser James Jones, have called on Obama to approve the pipeline. Jones told Congress last month that approval would send Russian President Vladimir Putin a message that "international bullies" can't use energy security as a weapon.
• The pipeline also is supported by labor unions eager for the jobs it would create, as well as a range of business groups and virtually all congressional Republicans.
• Environmental groups and some top Democratic donors oppose the pipeline, saying it would carry "dirty oil" that contributes to global warming. They also worry about possible spills.
• Tom Steyer, a billionaire environmentalist, has vowed to spend $100 million --$50 million of his own money and $50 million from other donors -- to make climate change a top-tier issue in the 2014 elections. Opposition to Keystone XL is a significant part of that effort.
• At least one environmental group said Thursday it would look to hold Senate Democrats "accountable" if the pipeline is approved. The group, 350 Action, has

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