Demonstrators protest against the proposed Keystone XL oil pipeline Monday in San Francisco. (Stephen Lam/Reuters)

The completion of a final State Department environmental analysis of the Keystone XL oil pipeline has thrust the issue into the middle of the 2014 elections, forcing President Obama to weigh his legacy on climate change against the reelection prospects of vulnerable Senate Democrats.

Both sides are already ratcheting up pressure on the White House. Environmentalists this week are flooding Secretary of State John F. Kerry’s office with calls and e-mails after holding nearly 300 candlelight vigils across the country Monday night in opposition to the project.

On the other side, four Democrats — Sens. Mary Landrieu (La.), Heidi Heitkamp (N.D.) and Joe Manchin III (W.Va.) and Rep. John Barrow (Ga.) — joined several Republicans, led by Sen. John Hoeven (N.D.), at a news conference Tuesday calling on the administration to approve a permit that would allow the project to move ahead.

“This pipeline is essential,” Landrieu said, adding that the United States already boasts 2.6 million miles of pipeline. “The time for study is over.”

The 1,179-mile northern leg of the Keystone pipeline would carry heavy crude from Canada’s oil-sands region in Alberta to Nebraska, allowing it to then be carried to refineries on the Gulf Coast. The State Department said in a final environmental assessment released last week that Keystone would be unlikely to alter global greenhouse gas emissions, because rejecting the project would not change the pace of energy development in the region.

Tuesday’s media event also featured officials from three labor unions — another key Democratic constituency — who back the pipeline because of the construction jobs it would provide. Jeffrey Soth, legislative and political director for the International Union of Operating Engineers, said construction on the southern leg of the project from Steele City, Neb., to Port Arthur, Tex., created the equivalent of about 1,000 jobs over a one-year period.

“We’re anxious to build more of this necessary infrastructure project,” Soth said.

Opponents say that, despite its conclusions, the State Department report illustrates the negative climate impact of allowing fossil fuels to be extracted and shipped through the pipeline. The report says that the 830,000 barrels of oil moving through the pipeline each day, when burned, would release the same amount of carbon dioxide as up to 5.7 million cars would annually.

Bill McKibben, co-founder of the environmental group, said the assessment has “everything in it that John Kerry and Barack Obama need to put this pipeline to rest forever.”

Environmentalists have made it clear they view the pipeline as a major litmus test for Obama, who pledged in June that he would approve it only if it “does not significantly exacerbate the climate problem.”

Opponents held 280 vigils in 49 states Monday night, according to Becky Bond, political director for the mobile-phone company Credo. The firm, one of the leading groups organizing the push against the project, e-mailed files that activists could print out for signs and urged them to buy candles for the events.

“For progressives, this was the biggest in-person rapid-response mobilization targeting President Obama during his two terms in office,” Bond wrote in an e-mail. “These are the people who could be mobilizing to defeat tea party Republicans in the mid-terms. But until the president rejects the Keystone XL pipeline all these people, all this energy is directed at the president and our fight to convince him to take urgent action on climate.”

Some lawmakers, such as Landrieu, may soon find themselves under fire as well. NextGen Climate Action, a political committee funded by billionaire Tom Steyer, is preparing to poll its supporters online to determine which politicians to target in this year’s election. “Tell Senator Mary Landrieu to Get Her Facts Straight,” one ad on the ballot reads.

The American Petroleum Institute, for its part, is already planning events in several states, including Iowa, Montana, New Hampshire, North Carolina and South Carolina, to highlight the benefits of the project. API President Jack Gerard said the group aims to show that the project “impacts every state,” including some where there are high-profile Senate contests.

There is considerable bipartisan support overall for Keystone, though not among liberals. A Pew Research poll from September found that 65 percent of Americans support the project and 30 percent oppose it. But among liberal Democrats, opposition outweighs support 54 percent to 41 percent, the poll showed. A Washington Post poll in June 2012 found similar results.

Kerry is just beginning to tackle the issue. On Monday, he started reading the environmental assessment, which runs more than 2,000 pages, according to a State Department official who requested anonymity in order to discuss internal agency deliberations. Kerry will also be briefed on the issue by senior State Department experts “very soon,” the official said.

Financial analysts remain divided on whether the administration will approve a permit application for TransCanada, the firm building the pipeline.

Barclays issued a report Monday saying that the State Department’s new analysis “does not, in our view, necessarily expedite the approval of the proposed Keystone XL permit; in fact, we believe that the final decision-making process will likely be protracted and final presidential approval remains in doubt.”

The Eurasia Group, by contrast, issued an analysis Tuesday saying there is “a 75% likelihood that the Keystone XL project is set for approval by President Obama later this year, although risks of foot dragging until after November midterm elections cannot be ruled out.”

Heitkamp said at the Capitol Hill news conference that it is important that a final decision be made within the next few months. “We cannot miss another construction season,” she said.

But members of the bipartisan group had no immediate plans to force the president’s hand, and Landrieu declined to speculate on the outcome of TransCanada’s permit application. “I don’t have a crystal ball,” she said, “and I am not going to predict what the president is going to do.”

Steven Mufson and Scott Clement contributed to this report.