WASHINGTON — Senate Republicans are expected to take a procedural vote Tuesday that, if it passes, would clear the way for lawmakers to begin debating a repeal of Obamacare.

"Big day for HealthCare. After 7 years of talking, we will soon see whether or not Republicans are willing to step up to the plate!" President Trump tweeted Tuesday.

But there are a lot of uncertainties associated with Tuesday’s “motion to proceed” vote, including what will be in the legislation and whether it can even pass with the Republicans' narrow majority.

No Democrats are expected to vote for the bill, so Republicans must rely on at least 50 of their 52 senators to back it. At various times in the process, more than a half-dozen Republicans have raised objections to repeal-and-replace plans.

The initial vote Tuesday will be to begin debate on the bill that the House of Representatives passed in May.

But Senate Republicans have not even considered the House bill, which President Trump called “mean.” If the House bill makes it onto the floor, senators will then offer amendments that would essentially replace the House bill.

“Everybody is going to get a vote on amendments, we’ll vote on everything,” Sen. John Cornyn, R-Texas, who is in charge of gathering and tracking votes, told reporters Monday afternoon. But critics — particularly Democrats, who are essentially spectators in this process — pointed out that amendments could vary drastically and senators were being asked to bring a bill to the floor with no clarity on what it would look like in the end.

It is widely assumed that the final bill will be one of two options: a clean repeal with the goal of drafting a replacement later or a repeal plan that includes a replacement.

House and Senate Democrats approved a repeal of Obamacare in 2015, and conservatives are requesting that legislation be the framework for this vote. But that vote was largely symbolic as lawmakers knew then-president Barack Obama would veto it.

McConnell and other Republican leaders have spent months crafting the repeal-and-replace plan also known as the Better Care Reconciliation Act, but even that may not garner enough support. Moderate Republicans have criticized drafts of the BCRA for going too far in rolling back the Affordable Care Act, while conservatives have said it doesn’t go far enough.

Maine Sen. Susan Collins, known as one of the most moderate Republicans in the Senate, is expected to be a “no” on any legislation that was not developed through committee hearings and bipartisan discussions — a request she acknowledged was unlikely.

“I’m very concerned about proceeding to the House bill, the first version of the Senate bill, the second version of the Senate bill, the 2015 bill to repeal and perhaps in two years come up with a replacement,” Collins told reporters Monday night. She was the only GOP senator still in the Senate who voted against the 2015 repeal bill. Collins said she would vote in favor of a motion to refer the bill to key committees for discussion, a motion she acknowledged was not on the table.

Collins seems to have been counted out by GOP leadership. The Maine senator told reporters she hadn’t heard from them in a couple of days.

Sen. Susan Collins arrives on Capitol Hill on July 25, 2017. (Photo: Mark Wilson, Getty Images)
Meanwhile, conservative Sen. Rand Paul, R-Ky., has said he won’t vote for any version of the repeal-and-replace legislation, which he has called “an enormous insurance bailout.”

On Monday, Paul told reporters he will vote for a motion to proceed only if it's clear that the Senate is proceeding to a straight repeal bill.

"I'm for going to a bill," Paul said. "I'm not for 'we're not sure what we're going to.' "

But there was a bright spot for Republicans on Monday night when Sen. John McCain announced that he would return to Washington following a brain cancer diagnosis last week. The Arizona Republican has criticized parts of the bill and the process for drafting it, but he's expected to vote with his party to bring the legislation to the floor.

Tennessee Sen. Bob Corker said that listening to his colleagues talk on the Senate floor Monday evening gave him hope for the legislation: “The direction that they’re taking sounds very promising … I went in without a sense of where we are and came out thinking maybe we’ve made some progress.”

But not everyone had the same sense of progress. Missouri Sen. Roy Blunt said that even if there aren't enough votes for a motion to proceed, it would be beneficial to hold a vote.

"After all the time now that we spent on this ... it's helpful for members to be able to show exactly where they would have been as opposed to try to explain that some other way," Blunt said.

Contributing: Erin Kelly