The White House on Thursday was quick to slap down a letter from 11 Senate Democrats that urged a presidential decision by May 31 on whether to build the hugely controversial Keystone XL pipeline.

But that doesn't relieve President Obama of pressure from Democrats, such as the letter's signers, who support building the pipeline.

What makes the letter still significant is that five of its signers are vulnerable senators who face very tough elections this fall. Republicans, who support the pipeline for reasons of jobs and energy security, need only six seats to take control of the Senate.

The political calculation for the president is this: Reject Keystone and endanger control of the Senate, or approve the pipeline and anger his base, which embraces green energy?

The letter from the 11 Democrats, sent Thursday, was spearheaded by Sens. Heidi Heitkamp of North Dakota and Mary Landrieu of Louisiana, both from energy states. Senator Heitkamp has been a fierce champion of the pipeline, which would carry crude oil from the tar sands of Alberta, Canada, to Gulf Coast refineries in the United States.

In February, Heitkamp brought together a bipartisan group of House and Senate members to push for presidential approval of the pipeline. Senator Landrieu, as the new chairman of the Senate Committee on Energy and Natural Resources, is working on legislation to move forward on Keystone and is busy gathering 60 votes to beat any filibuster. She plans to hold a hearing in the next few weeks.

Congressional historian Julian Zelizer, of Princeton University, says the politics favors Mr. Obama eventually siding with the vulnerable senators. “This is an issue where he’s been very hesitant to put Democrats and himself at risk," he says.

With his years of delay on a decision, “he’s been willing to anger environmentalists over and over again on this issue,” Mr. Zeliger says. One sign of that: Obama expedited construction on the southern part of the pipeline by pushing along the permitting process.

On the other hand, environmentalists, ranchers, native Americans, and now Latinos, according to one recent poll, vigorously oppose building the pipeline. They decry it for reasons of climate change, other environmental degradation, and more reliance on dirty oil.

Tom Steyer, a San Francisco billionaire and Democrat, is putting up $50 million of his own money to support candidates fighting global warming. His advocacy group, NextGen Political Action, plans to raise another $50 million to match that. The funds, however, will not be spent against Democrats who duck or oppose global warming, though it will be withheld from them.

In their letter, the Senate Democrats were clear: no more waffling. They asked for a presidential decision no later than May 31. “This decision must not drag on into the summer,” risking another construction season, wrote Heitkamp and Landrieu. Co-signers Mark Begich of Alaska, Kay Hagan of North Carolina, Mark Pryor of Arkansas, and John Walsh of Montana also face tough elections this fall.

The senators argued that enough time has been spent reviewing the decision. “This is a process that has now gone on well past five years, has involved two applications, five federal reviews, multiple open comment periods, and numerous opportunities for consultation and comment,” their letter said.

In January, the State Department’s environmental impact review concluded that the pipeline would not pose a significant environmental threat. That review, the senators wrote, “reached virtually the same conclusion as previous reviews, that construction of the Keystone XL pipeline is ‘unlikely to significantly impact the rate of extraction in the oil sands or the continued demand for heavy crude oil at refineries in the United States.’ ”

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They urged Obama to make the “right” decision and said that building the Keystone XL pipeline is in the national interest. But White House spokesman Jay Carney said the president would stick by the process and wait for a recommendation from the State Department before making his own decision.

For opponents of the pipeline, that still leaves hope. Later this month, an alliance of ranchers, farmers, and native Americans are bringing their horses and tepees to the National Mall in Washington. Their encampment in opposition to the pipeline is expected to last a week – a reminder in the president's own backyard that he once vowed to end "the tyranny of oil."