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They have warned Mr. Obama of political peril in November’s elections if he approves the project.

The Sierra Club sent a frantic email Tuesday evening asking supporters to call their senators and rally in opposition to the pipeline.

“It would ensure more disastrous oil spills, threaten sources of drinking water for millions, disrupt wildlife, and increase rates of cancer and other health problems in Canada and in refinery communities here in the United States,” the club said.

Supporters of the pipeline say the Obama administration’s State Department has concluded that the pipeline won’t increase greenhouse gas emissions because, among other reasons, Canada has said it will mine and sell the oil no matter what, possibly to U.S. customers shipped through railroad cars.

Supporters also say the Keystone XL pipeline would create thousands of construction jobs and provide energy from a stable and friendly neighbor rather than vulnerable and sometimes hostile regimes in the Middle East and Venezuela.

Red-state Democrats have been among the most vocal in demanding that Mr. Obama speed up approval.

Ms. Landrieu and Sen. John Hoeven, North Dakota Republican, are lead sponsors of a stand-alone Keystone approval bill.

It has 11 Democratic co-sponsors, including five others facing election fights besides Ms. Landrieu: Mark L. Pryor of Arkansas, Kay R. Hagan of North Carolina, Mark Begich of Alaska, Mark R. Warner of Virginia, and John E. Walsh of Montana.

“It’s a hell of a pickle for Harry,” said Michael McKenna, a Republican energy policy strategist. “There’s no good way to work it. The problem is that there are some Democrats who really need a vote on Keystone and other Democrats who really, really, really want to avoid it.”

He said there was no obvious way for Mr. Reid to “square the circle,” and it likely would doom the entire energy bill.

“He can’t help some without hurting others,” Mr. McKenna said.