WASHINGTON — A group of Democratic senators, including several facing tough re-election races, sent a letter Thursday to President Barack Obama demanding approval of the Keystone XL oil pipeline by May 31.

The letter’s forcefulness hints at the growing anxiety among many Democrats that they could lose control of the Senate in November. A decision by the end May to approve controversial $5.3 billion project would give embattled Democratic candidates who are running in more conservative states a timely accomplishment to tout to skeptical constituents.

“The time to act is now Mr. President,” the letter from 11 Senate Democrats says. “Please use your executive authority to expedite this process to a swift conclusion and a final decision so that we can all move forward on other energy infrastructure needs in this country. We ask that you bring this entire process to an end no later than May 31, 2014, and that your final decision be the right one, finding that the Keystone XL pipeline is in the national interest.”

The letter was initiated by Sens. Heidi Heitkamp of North Dakota and Mary Landrieu of Louisiana, the latter in statistical dead heat in the polls with her likely Republican challenger. They were joined by Sens. Mark Begich, D-Alaska, Mark Pryor, D-Ark., Joe Manchin, D-W.Va., Joe Donnelly, D-Ind., Claire McCaskill, D-Mo., Mark Warner, D-Va., Jon Tester, D-Mont., John Walsh, D-Mont., and Kay Hagan, D-N.C.

Like Landrieu, Begich, Pryor, Walsh and Hagan are running in very close races in conservative states. Republicans need to gain seats to win control of the Senate.

The Keystone XL pipeline would run about 1,200 miles from Hardisty, in Alberta, Canada, to Steele City, Neb., where it would link to a southern portion to take the oil to refineries along the Texas Gulf Coast. Because it crosses a United States border, the pipeline needs a so-called presidential permit from the State Department.

The pipeline has become an unexpected flashpoint, a symbol of conflicting concerns about job creation and energy reliability on the one hand, and environmental protection and climate change on the other. In light of the controversy, the president has said he would make the final decision about the permit.

The senators are “thinking obviously that this letter demonstrates a commitment to growth and jobs and that the president is not being beholden to national environmental groups,” said Stuart Rothenberg, editor of The Rothenberg Political Report, a nonpartisan newsletter covering federal elections.

Republicans, much of the business community, some unions and conservative Democrats have backed the pipeline, arguing that it makes sense to import more oil from Canada than from unstable, unfriendly regimes. They have also argued that the project would create thousands of jobs. Keystone XL would create several thousand direct jobs during construction but fewer than 50 full-time jobs once it is running.

Environmentalists, many climate scientists, faith leaders, some communities along the proposed route and much of the Democratic base oppose Keystone XL, arguing that it would lead to the development of fossil fuel deposits in Canada that would spew more greenhouse gases into the atmosphere and worsen climate change.

The application for Keystone XL was submitted more than five years ago and is now being reviewed by the State Department and eight other federal agencies to determine whether building it would be in the national interest. The agencies have until May 1 to make their recommendation to Secretary of State John F. Kerry.

But there is no deadline after the recommendation is delivered for the secretary and the president to make their decisions, the subject of the letter’s concerns.

White House press secretary Jay Carney said the White House would not speed up deliberations to meet the senators’ demands.

“Our position on that process hasn’t changed, which is that it needs to run its appropriate course without interference from the White House or Congress,” Carney said at the daily briefing. “So that review continues at the State Department where it’s housed in accordance with past practice of previous administrations of both parties. And when there’s a decision to be announced, it will be announced.”

Analysts had differing opinions about whether the Obama administration’s approval of the permit would help vulnerable Democrats.

Jennifer Duffy, senior editor of The Cook Political Report, a nonpartisan newsletter, said that granting the permit could bolster senators from oil-producing states, like Landrieu, Begich and Walsh, by helping the administration look friendlier to fossil fuels. Keystone XL would only directly affect Montana, however, where Walsh is a candidate.

But Rothenberg said that granting a permit would do little to boost Obama’s popularity in the states where the races are tightest, none of which he won in 2012, and it could infuriate the Democratic base.

“It would produce a firestorm among environmentalists and focus the midterms on divisions within the Democratic Party,” he said. “Right now, the Democrats are trying to make sure that the base is energized and if you drop a bomb like that in laps of their base, there would be a lot of moaning, griping, backbiting and sniping all the way to the election.”

Nearly half of Democrats favor granting a permit, according to a March poll by the Pew Research Center for the People &the Press. But the most devoted voters who would be expected to turn out for the midterm are staunchly against the pipeline.

The Obama administration’s decision, whatever it turns out to be, could be rendered moot by a pending lawsuit in Nebraska, through which the pipeline would run. A February ruling by a Nebraska court invalidated the pipeline route the state had approved, threatening a further delay of the project. That decision has been appealed to the state Supreme Court.

“The department continues to review the presidential permit application for the proposed Keystone project,” a State Department spokesman said. “We are monitoring the ongoing litigation in Nebraska.”