This item updated at 10:55 p.m.

For the first time in the six-year fight over the controversial Keystone XL pipeline, both houses of Congress will hold a vote on the proposed project, giving each side in a Louisiana Senate election a chance to boost its candidate.

The two lawmakers locked in the runoff contest, Sen. Mary Landrieu (D) and Rep. Bill Cassidy (R), seized control of the congressional agenda Wednesday, extracting assurances from House and Senate leaders that votes will be held to bypass President Obama’s authority and authorize construction of the pipeline.

A large showing of Democratic support for the pipeline could complicate the administration’s decision-making process, given the party’s dismal showing at the polls last week. Environmentalist allies of the president are solidly against the project and have been doggedly lobbying the administration against approving it.

The first thing to know about the Keystone pipeline? It already exists. Here's a breakdown of the pipeline's various parts. (Gillian Brockell, Jhaan Elker and Kate M. Tobey/The Washington Post)

But Republicans successfully used the president’s environmental and climate agenda as key lines of attack against Democrats in several contested midterm races. Those results strengthen the arguments of those who believe that it would be a political mistake for the administration to deny permits for the unbuilt sections of the pipeline, and congressional approval of the project could put the administration on the defensive if it were inclined to halt the project.

Acknowledging the importance of energy to Louisiana’s economy, Landrieu and Cassidy have championed completion of the pipeline, which would transport oil from the tar sands of Canada to the Texas Gulf Coast. The GOP-controlled House voted several times in recent years to support the pipeline, while the Senate, in deference to the administration’s review, has resisted holding a vote on the matter despite strong objections from several moderate Democratic senators from rural or energy-rich states.

For six years, the pipeline has been under review by the State Department, which has jurisdiction because the project crosses international borders. Democrats such as Landrieu from energy-producing states have joined Republicans in calling for its approval.

Sen. Joe Manchin III (D-W.Va.) voiced strong support for the plan on Wednesday, saying that “it would be a tremendous windfall for all of us. It’s something we can count on. And I can’t for the life of me understand why we haven’t to date been able to move this piece of legislation forward.”

A Senate vote next week will allow Landrieu to say that she voted for the project, putting some distance between her and Obama.

Whether Congress can authorize constructing the pipeline has been the source of debate between Republicans pining for its construction and environmental groups who say the powers rest solely with Obama.

But supporters say that Congress can use its power to regulate commerce with foreign countries to authorize the project. The legislation, as written, would authorize constructing the pipeline and use a January 2014 environmental-impact report by the State Department to satisfy federal requirements that the project be studied for adverse effects.

After their midterm losses, there is little political fallout for Senate Democrats to worry about. Landrieu, Manchin and at least nine other Democrats support building the pipeline, and Republicans are expected to make up the bulk of the votes approving the plan in the Senate next week. Even if it doesn’t pass this year, the GOP has long vowed to approve the pipeline once they win total control of Congress.

But in recent days, several Democrats and Republicans have cited authorization of the oil pipeline as a modest proposal that could be used to restore bipartisan cooperation in the fractured Congress. Party leaders agreed suggesting that it could be voted on next year in the new Congress.

Landrieu had other ideas.

“I don’t think we necessarily need to wait until January,” she said Wednesday in a floor speech that lasted almost three hours. Landrieu made no attempt to hide her motive. “I’m going to do everything in my power here and at home on the campaign trail, where I’m still in a runoff, as you know, to get this project moving forward,” she said.

And later, she secured an agreement from Democratic and Republican leaders to hold a vote authorizing the pipeline as early as next Tuesday. House Republican leaders also announced plans to hold a vote as early as Thursday to authorize the pipeline, the ninth time the GOP-controlled House has voted to approve the pipeline in the past six years.

Before her remarks, Landrieu was spotted riding the escalator alone up from the Senate trains that carry lawmakers between their offices and the Capitol, toward a row of elevators. She was stone-faced and declined to answer questions from reporters. Once she reached the top level and stepped off, Sen. Charles E. Schumer (D-N.Y.), one of his party’s top campaign strategists, walked over.

Smiling, he asked Landrieu to step aside for a private conversation. She shook her head and moved briskly toward the elevator. As she did, she pointed to her phone, saying she had a call. Schumer paused for a moment as she moved away. His smile dropped, and he turned to follow her. “Mary, Mary,” he said, a few steps behind, asking her to speak with him. When she kept moving and ducked into an elevator, he hustled and jumped in to join her as the doors closed.

A few minutes later, Landrieu took to the Senate floor to vent her frustrations and to try to shift the political winds in her direction.

Robert Costa contributed to this report.