A key vote to advance legislation green-lighting the Keystone XL oil sands pipeline failed 53-39 in the Senate on Monday.
The vote aimed at cutting off debate on legislation to approve the controversial project fell short of 60 votes after Democrats, angry at Republicans for blocking debate on a slate of Democratic amendments last week, blocked the measure.
"Last Thursday night the majority decided that they would not allow for debate," Democratic Sen. Ed Markey said on the floor ahead of the vote, echoing a complaint that a number of Democrats expressed on Monday after the Senate reconvened to debate the bill.
Senate Republicans have instigated a "gag-a-thon," said Sen. Barbara Boxer, D-Calif.
Democratic Sen. Tom Carper of Delaware said he voted against ending debate because of the "failure of the majority to follow through on the open amendment process," taking aim at Senate Majority Leader Mitch McConnell.
Shortly before midnight on Thursday, McConnell filed a motion to invoke cloture and limit debate on the bill, setting up an expected vote this week.
"I truly hope...Democrats won't vote to block Keystone's jobs now, just because a different party controls the Senate," McConnell said ahead of the vote.
Democratic opposition now throws into question the timing of a final vote, which may not come until next week.
Tensions brewing between Democrats and Republicans during the KXL debate could offer a preview of what's to come in the new Congress.
Republican Sen. Lisa Murkowski of Alaska expressed optimism that Democrats and Republicans could "work towards an agreement that would allow us to get to passage of the Keystone XL pipeline," after the vote and said that she would work with Democratic Sen. Maria Cantwell of Washington to define a list of amendments that could be considered during the KXL debate.
Democratic Sen. Chuck Schumer of New York told reporters in the Capitol that Democrats will move forward on the bill if McConnell assures an open amendment process.
"That's the way you gotta roll," Murkowski said after the vote. "But I hope we will be able to get back on track."
A second attempt at advancing pro-KXL legislation failed by 53-39 after the initial vote. Debate over the KXL bill has been underway in the Senate since the beginning of January.
This story is breaking and will be updated.
Jason Plautz contributed to this article