WASHINGTON — The runoff election for the US Senate seat from Louisiana has jump-started long-stalled legislation to build the Keystone XL oil pipeline.
On Wednesday, Senate Democrats abandoned efforts to block the measure in hopes of helping endangered Senator Mary Landrieu keep her seat in the energy-rich Bayou State.
Republicans responded swiftly by scheduling a vote in the House on Thursday on an identical bill sponsored by Representative Bill Cassidy, Landrieu’s Republican rival in the Dec. 6 runoff.
It was unclear how the votes would affect that race, but Senate passage of the bill as early as next Tuesday would force President Obama to either sign the measure into law or veto it just weeks after a Democratic drubbing in midterm elections.
Republicans and several moderate Democrats insist that construction of the Canada-to-Texas pipeline would create tens of thousands of jobs. Opponents maintain that the project would hurt the environment and contribute to climate change.
The White House had no immediate comment on the day’s developments.
‘‘I believe that we should take the new majority leader at his word and stop blocking legislation that is broadly supported by the American public and has been for quite some time,’’ Landrieu said in a speech on the Senate floor. ‘‘I want to say ‘yes’ to majority leader — new majority leader Mitch McConnell. The time to start is now.”
Landrieu cast herself as someone willing to challenge Democrats and Republicans.
‘‘I’ve stood against my leadership,’’ she told reporters, and added, ‘‘and I’ve stood up to the Republicans.’’
The back-and-forth came against the backdrop of a new political landscape and fresh calls for an end to Washington gridlock. In the midterm elections, Republicans seized majority control of the Senate with a net gain of eight seats. Cassidy is heavily favored in the runoff, and his victory would give Republicans a 54-to-46 majority.
McConnell said the election of a Republican Senate majority has already changed the dynamic.
‘‘I hope this post-election conversion on Keystone signals Democrat cooperation on a whole host of other energy bills they have blocked, and whose passage would help to make America more energy-independent,’’ he said in a statement.
Echoing Landrieu’s plea for a vote were moderate Democrats from Republican states, who argued that the project that would carry oil from Canada south to the Gulf Coast. The southern leg of the pipeline between Oklahoma and Texas is already operational.
Senate Majority Leader Harry Reid, a Nevada Democrat, has blocked the Keystone measure, leading to Republican claims that Landrieu is ineffective as chairman of the Energy and Natural Resources Committee.
A senior Senate Democratic aide said Reid would not attempt to defeat Landrieu’s effort. Asked if Obama would sign it, Landrieu told reporters: ‘‘I do not know.’’
Energy has been a central issue in the Senate race, and Keystone a frequent flashpoint with both Landrieu and Cassidy supporting the project. Cassidy has said Landrieu has been unable to deliver because of her party’s leadership.
Landrieu has a strong alliance with the oil and gas industry and has pushed for an expanded drilling in the United States.
If elected, Cassidy would get a seat on the energy panel. As a new senator, he would be low in the pecking order of panel members, and Cassidy and Louisiana’s all-GOP congressional delegation would likely have little sway with the Obama administration.
As Louisiana’s last Democratic officials elected statewide, Landrieu has a difficult path to victory in a state that overwhelmingly backed Mitt Romney in 2012. Fifty-eight percent of voters supported someone other than Landrieu in the primary last week.
Republican leaders are uniting behind Cassidy, a three-term congressman, while the national Democratic Party has decided not to provide advertising support for Landrieu in the runoff.
Asked if she was a lost cause, Landrieu told reporters, ‘‘No, I don’t believe that I am.’’
In a statement, Cassidy said it ‘‘is easy to wonder if the Senate is only considering this because of politics, even so, I hope the Senate and the president do the right thing and pass this legislation creating thousands of jobs.’’