Republicans on Tuesday cleared a crucial hurdle to begin the Senate healthcare debate, but now face the danger of a circular firing squad if they are unable to unify around a single proposal.

The successful procedural vote sets up 20 hours of floor debate and votes on a virtually unlimited number of amendments, known as a vote-a-rama.

Many of these amendments will fail, and some will pit centrists in the GOP conference against conservatives, a division that has made it exceedingly difficult for Republicans to move forward on ObamaCare repeal.

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At press time, votes were expected Tuesday night on an amendment to repeal ObamaCare in two years, and one to repeal and replace ObamaCare with a substitute that Senate Republicans have been working on behind closed doors for months.

Both of those amendments are widely expected to fail, but GOP leaders think it will help them to determine how much support exists for both measures.

That information could be used to craft another compromise before the underlying healthcare bill comes up for a final vote.

Another possibility that seemed to have growing momentum on Tuesday is a so-called “skinny” ObamaCare repeal bill that would only eliminate the healthcare law’s insurance mandates on individuals and businesses and an unpopular tax on medical devices.

If that legislation can pass the Senate, it could be brought to a conference negotiation with the House, when lawmakers from both chambers would try to reconcile their differing products.

Yet it is far from clear that even the “skinny” measure could win support.

Sen. Susan Collins Susan Margaret CollinsGraham: GOP will confirm Trump's Supreme Court nominee before the election Gardner signals support for taking up Supreme Court nominee this year Tumultuous court battle upends fight for Senate MORE (R-Maine), long a holdout from her party in the healthcare debate, expressed skepticism about the approach on Tuesday. Sen. John McCain John Sidney McCainBiden's six best bets in 2016 Trump states Replacing Justice Ginsburg could depend on Arizona's next senator The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE (R-Ariz.) predicted in a dramatic floor speech after his return to the Senate following a brain cancer diagnosis that the entire effort was likely to fail, and that Republicans should start over with an open process and committee hearings.

Democrats plan to make the process as painful as possible for Republicans by dragging out proceedings. No Democrats backed the measure to begin debate, and they withheld their “no” votes until the end to highlight the contrast to the GOP.

On Tuesday evening, Democrats forced the Senate clerk to read the entire first proposed amendment to the bill, which was expected to take a couple of hours.

“The aim of our amendment strategy is to defeat the various versions of repeal they proposed and to make it as hard as possible to get to 50,” said a senior Democratic aide.

Republicans control 52 seats, meaning they can survive only two defections with Democrats unified and Vice President Pence breaking a tie.

Several moderates, including Collins and Sens. Shelley Moore Capito Shelley Wellons Moore CapitoCongress must finish work on popular conservation bill before time runs out Second GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure MORE (W.Va.) and Lisa Murkowski Lisa Ann MurkowskiGraham: GOP will confirm Trump's Supreme Court nominee before the election Gardner signals support for taking up Supreme Court nominee this year Tumultuous court battle upends fight for Senate MORE (Alaska), say they will not support a repeal measure that leaves the question of replacement unanswered.

Republicans are likely to face a rollicking debate over as many as 100 different healthcare proposals.

GOP leaders describe the “skinny” option as a fallback proposal.

“Who knows what the final bill will look like. I’d be happy to have a comprehensive bill that 50-plus senators agree to, but if we can’t, then the idea would be to come up with a core of pieces that 50 of us agree on,” Senate Majority Whip John Cornyn John CornynTumultuous court battle upends fight for Senate Texas Democrats roll out first wave of planned digital ads as Election Day nears Calls grow for Biden to expand election map in final sprint MORE (R-Texas) told reporters.

Even Republican leaders admitted they don’t fully know what to expect.

“There will be a lot of different amendments offered by different members trying to craft the bill. It’s really entirely impossible to predict, in a reconciliation debate, exactly what amendments will be offered or what amendments will succeed. It’s wide open,” Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellGraham: GOP will confirm Trump's Supreme Court nominee before the election Trump puts Supreme Court fight at center of Ohio rally The Memo: Dems face balancing act on SCOTUS fight MORE (R-Ky.) told reporters.

Cornyn predicted that as many as 100 different healthcare proposals could receive votes.

“Under the budget process, everybody is entitled to get a vote on an amendment if they want one. So it’s impossible to predict the sequence,” he said. “We’re going to be doing a lot of voting this week. I hope you all have eaten your Cheerios.”

Democratic Sen. Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE (Conn.) told reporters that he alone has prepared more than 100 amendments for floor consideration.

The Budget Committee will vet the various amendments to make sure they comply with Senate rules.

The advantage of passing healthcare legislation under special budgetary rules is that GOP leaders can pass it with a simple majority, instead of 60 votes as is usually required for controversial legislation in the Senate.

But legislative provisions must pass a six-part test known as the Byrd Rule. The most stringent requirement is that the budgetary impact of the proposals must be more than incidental compared to the policy impact.

In other words, policymaking that does not have a significant impact on spending, revenues or the deficit is not allowed.

Nathaniel Weixel contributed.