Senators are gearing up to pull an all-nighter as they head toward a showdown on Republicans’ years-long pledge to repeal and replace ObamaCare.

Lawmakers are expected to spend hours on the Senate floor Thursday night and into Friday morning as they sort through hundreds of amendments ahead of a final vote on their healthcare effort.

Senators are able to use the free-wheeling marathon session, known as a vote-a-rama, to force a vote on any amendment they want. The hours-long floor fights are frequently used to make members on the other side of the aisle take politically tough votes, creating fodder for the upcoming 2018 midterm election and 2020 election.

Marathon sessions frequently stretch into the early morning hours. A vote-a-rama in January that set up the GOP's ability to fast-track an ObamaCare repeal ended after 1 a.m., while another vote-a-rama in 2015 wrapped up after 3 a.m.

The GOP endgame — and whether there were enough votes to get there — remained unclear even in the hours leading up to the chaotic floor drama.

Republican senators appear to be coalescing around passing a “skinny repeal” of ObamaCare, and GOP leadership filed its plan late Thursday night.

The measure — which would be attached to the House-passed bill that is being used as a vehicle for any Senate action — is expected to include a one-year defunding of Planned Parenthood, a repeal of ObamaCare's individual insurance mandate and at least partial repeal of the employer mandate.

But several GOP senators want an assurance that House Republicans will agree to a conference between the two chambers before the Senate has to take its final vote.

"There's increasing concern on my part and others that what the House will do is take whatever we pass" and pass it without making changes, Graham said. "The 'skinny bill' as policy is a disaster. The 'skinny bill' as a replacement is a fraud.”

" Until the Senate can do that, we will never be able to develop a conference report that becomes law. We expect the Senate to act first on whatever the conference committee produces," he said.

McCain appeared unconvinced ahead of the pair of 8:30 p.m. votes, telling reporters that he is "not satisfied."

Sen. James Lankford (R-Okla.) predicted that lawmakers could be waiting until September to get the CBO’s analysis for all of their proposals, hence why Republicans needed to go to conference and buy themselves more time.

The GOP's repeal proposal will face a key test shortly after midnight. Democrats will try to send it to a Senate committee, which would effectively pigeonhole the amendment. If that move fails, as it's expected to, Senate Republicans will vote on attaching it to the House-passed healthcare bill, which is being used as a vehicle for the Senate legislation.

As of 7 p.m. , fewer than 200 amendments had been filed to the bill, with senators able to continue filing their proposals throughout the night.

Democrats have warned they are holding off on offering amendments until Senate Republicans unveil their healthcare endgame.

The New York Democrat signaled that his caucus will wait until after Republicans vote on the “skinny repeal” amendment to offer their own suggestions.

“I want to put my colleagues on both sides of the aisle on notice: My Democratic and Republican colleagues that they should prepare for numerous Democratic amendments if the skinny bill passes. ... It won’t be the last vote,” he said.

Democrats also want to use the Senate's rulebook to try to stymie GOP proposals, by forcing them to meet 60-vote thresholds and keep their focus on trying on closed-door process to crafting the GOP healthcare bill.