With a handful of senators out and Democrats calling for more amendments, GOP leaders were unable to limit debate on a measure to approve the Keystone XL pipeline Monday.

By separate votes of 53-39, the Senate rejected two cloture motions on a substitute amendment that would approve the pipeline and on the nearly-identical underlying bill (S 1). But it appeared that negotiations on additional amendments would occur and movement on the legislation would continue.

Four Democrats — Michael Bennet of Colorado, Joe Donnelly of Indiana, Heidi Heitkamp of North Dakota and Joe Manchin III of West Virginia — voted in support of the debate-limiting motions. Eight senators also were absent from Monday’s session for a variety of reasons, from campaign commitments to the flu.

Majority Leader Mitch McConnell, R-Ky., switched his vote on both motions to “no” so that he could make a motion to reconsider the vote at a later time.

After the vote, Energy and Natural Resources Chairwoman Lisa Murkowski, R-Alaska, said senators are working on a list of amendments that could be whittled down for additional votes and move the chamber toward passage. She noted that more than 140 amendments have been filed to the measure.

“There’s still much to be done with this bill if your interest is voting on amendments,” she said.

Senate Republicans have said they plan to conference their bill with a House-passed measure (HR 3) that would simply approve the pipeline. That bill was not amended.

McConnell indicated earlier in the day that he expected the filibuster, suggesting Democratic leaders pressured rank-and-file members to block the motions after amendment negotiations fell apart on the floor last week.

McConnell said nine Democrats voted for the Keystone project last year without the opportunity to debate a single amendment.

“A Keystone filibuster cannot succeed without the support of Democrats who voted for a Keystone bill just a few weeks ago without any amendments, and who are co-sponsoring this jobs bill today,” he said.

North Dakota Democrat Heidi Heitkamp, who voted to limit debate, said she wants to move on to other energy issues but acknowledged the discontent among her party colleagues.

“Would this vote have been different had the process been different on Thursday?” she said. “Yeah, this vote would have been different had the process been different, and that’s disappointing.”

Democratic Leader Harry Reid, D-Nev., weighed in after he recovered from eye surgery earlier in the day.

“I have never seen debate shut down as aggressively as when Senator McConnell refused to allow Democratic senators to debate their own amendments for a single minute — and that’s saying something,” Reid said in a statement.

Assistant Democratic Leader Richard J. Durbin, D-Ill., said Democrats wanted to offer constructive proposals but last Thursday’s events were “troubling” and didn’t leave a good taste in the mouths of many in his party.

“If we’re going to have a good faith, bipartisan environment to consider amendments, let’s go back and forth — Democrat, Republican — let’s consider the major issues before us. And there are still major, unresolved issues, health and safety issues, with pending amendments,” he said.

But both leaders said there is still a chance for the consideration of more amendments.

Durbin said he told McConnell, “’Even if we don’t pass this cloture motion this evening, let’s work together on a bipartisan basis. Let’s come up with this list of amendments. Let’s do this in a conscientious, good faith effort to complete this bill, and I think we can achieve it.”

 Lauren Gardner contributed to this report.