But the fate of the $8 billion Canadian project depends not only on what happens in Congress, but also in the courts, at the White House and with Calgary-based TransCanada Corp., the pipeline's developer. TransCanada has been waiting more than six years for approval of the pipeline.
The odds of building seem to change almost daily.
A lawsuit is tossed out, then some others filed. The Republican-controlled Congress is poised to approve a bill authorizing construction, despite a White House promise of a veto. The State Department, after stalling its review because of a Nebraska court case, gives federal agencies a new deadline to weigh in.
The US $8-billion pipeline would carry an estimated 830,000 barrels of oilsands bitumen from Canada into the United States, connecting with existing pipelines leading to Gulf Coast refineries.
For environmentalists, approval would prove catastrophic for the global climate and erode efforts by President Barack Obama to rein in heat-trapping emissions.
For Republicans, who have made it their first order of business in the new Congress they control, the pipeline is critical to supplying the country with jobs and with oil from a friendly neighbour, rather than the Middle East.
A look at the major players and where they stand:
The House of Representatives this month passed legislation approving, for the 10th time, the pipeline's construction. Identical legislation cleared an initial hurdle in the Senate, where a 63-32 vote was three more than the 60 required, but not enough to override a veto.
The Senate is expected to vote to wrap up debate, after weighing in on more than a dozen amendments. Republicans and Democrats have so far only agreed to a few additions to the bill, including a 98-1 vote backing a measure saying climate change is not a hoax and real. But Republicans failed to back two other measures saying human beings played a role in global warming, even though the scientific consensus is that the burning fossil fuels is to blame. Late last week, the Senate backed a non-binding measure saying that oil sands should be subjected to a tax used to pay for oil spill cleanups.
THE WHITE HOUSE
The White House explained the veto threat by saying it had to wait for the outcome of a Nebraska court case and the State Department review to play out.
Obama's remarks about the pipeline have gotten increasingly negative in recent months, emboldening environmental groups who have called on him to reject the pipeline outright. In his State of the Union address Tuesday night, Obama urged Congress to "set our sights higher than a single oil pipeline."
Opponents in Nebraska reignited the legal fight last week, filing two new suits over its proposed route.
They came from seven landowners in Holt and York counties who have received written warnings that TransCanada plans to file eminent domain papers to gain access to their land.
The landowners' lawyer, Dave Domina, said TransCanada's written warnings give the plaintiffs a clear legal basis to challenge the project. That's a key issue, because the Nebraska Supreme Court tossed an earlier, similar suit, with three judges saying those plaintiffs failed to meet the threshold. The judges' decision removed an obstacle to the project and was a major reason the White House threatened a veto.
THE U.S. STATE DEPARTMENT
The department has given federal agencies until Feb. 2 to weigh in on whether the pipeline serves the national interest, a determination required for all border-crossing pipelines. That gives agencies specializing in the environment, commerce and other matters just weeks to file their opinions.
The department didn't set a timeline for when it would make its long-awaited judgment on whether the pipeline was in the U.S. national interest.
With the state court removing a legal barrier, Calgary-based TransCanada has filed paperwork in nine counties to acquire access to the remaining land that's needed to construct, operate and maintain the pipeline.
By law, TransCanada can use the courts to force landowners to sell access to their land. Company officials say they still need to acquire 12 per cent of the total land easements from landowners who have not yet reached a deal. Some holdouts have said they won't negotiate.
The route still faces legal challenges in Nebraska, where two new suits have been filed.