Eliza Collins

USA TODAY

WASHINGTON — Senate Majority Leader Mitch McConnell, R-Ky., will push ahead Tuesday with a key vote in the GOP's attempt to repeal Obamacare, an effort that may have gained momentum with Sen. John McCain's announcement that he would return to Washington.

"Look forward to returning to Senate tomorrow to continue work on health care reform, defense bill & #RussiaSanctions," McCain tweeted Monday night.

The return of the Arizona senator, who disclosed his brain cancer diagnosis last week, is crucial for Senate Republicans amassing enough votes to proceed with legislation to repeal Obamacare.

It is still not clear what will be in the Senate GOP bill or whether it has enough votes to pass.

"Many of us have waited literally years for this moment to arrive and, at long last, it finally has," McConnell said on the Senate floor Monday as he urged senators to vote Tuesday on a procedural motion to start debate on a health care bill.

"I will vote yes on the motion to proceed and I would urge all our colleagues to do the same," he continued.

Even if enough lawmakers back the legislation, the bill may not make it past the ultimate gatekeeper: the Senate parliamentarian.

Two different versions of legislation have been discussed.

The first option is the Better Care Reconciliation Act, which would repeal major parts of Obamacare and offer a replacement. It has been criticized by moderates for going too far in rolling back the Affordable Care Act and by conservatives for not going far enough.

Four Republican senators have already said they will vote against bringing this version of the legislation to the floor for debate; if more than two Republican senators vote no, the Senate will not be able to begin debate.

The second option is a repeal of the Affordable Care Act that would take effect at a future deadline. The goal is for lawmakers to have time to draft a replacement down the road, ideally before the effects of a repeal are felt. The House and Senate passed a version of this bill in 2015, but the vote was largely symbolic because lawmakers knew then-president Barack Obama would veto it.

Three senators have said they will not vote to proceed on legislation like this. Others have also expressed concern about repealing the health care law without a replacement ready.

Senate Majority Whip John Cornyn of Texas, who is in charge of securing votes for the GOP, said that the substance of the bill — which would be decided by amendments — will be "further discussed" during Tuesday's Senate Republican lunch.

Cornyn expressed confidence that lawmakers would approve the "motion to proceed" but acknowledged it was "a very narrow path."

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Conservative lawmakers and outside groups are calling on senators — who spent the last seven years railing against Obamacare — to keep their word and vote the same way they did in 2015.

About 20 conservative activists met with McConnell at the Capitol on Monday afternoon. The group, led by Jenny Beth Martin of Tea Party Patriots, brought a letter with 1.5 million signatures that demanded the Senate do a full repeal of Obamacare, Martin told USA TODAY.

"He indicated he’s working to get to 51 votes and that something is better than what we have now," Martin said when recounting the meeting with McConnell. But Martin, who does not support the BCRA, said she told McConnell that conservative activists “want full repeal and they want real repeal and they’ve been working this issue for nearly 8 years.”

When a reporter asked Sen. Rand Paul, R-Ky., on Monday evening whether he knew what would be in the bill, he joked, "No, I was hoping you would tell me."

Paul supports a simple repeal bill. He opposed the Republican bill to replace Obamacare as a giveaway to insurance companies.

He said 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.' "

Meanwhile, Trump is urging his party to pass something — anything, really.

On Monday, he tweeted that Republicans needed to pass a repeal-and-replace bill after campaigning on it for so long.

But last week, the president flipped between calling for a straight repeal and just letting Obamacare fail.

On Monday afternoon, Trump and Vice President Pence convened a meeting “with victims of Obamacare.”

After the meeting, Trump criticized both Republicans and Democrats over their inability to repeal the "big, fat, ugly lie" known as Obamacare.

"Any senator who votes against starting debate is telling America that you are fine with the Obamacare nightmare,” Trump said.

Democrats, meanwhile, attacked Republicans for their lack of transparency.

Minority Leader Chuck Schumer, D-N.Y., complained that "no one knows" which bill GOP leaders will bring to a vote.

"It's like Alice in Wonderland around here," he said on the Senate floor Monday.

Even if McConnell is able to secure enough votes to squeak through the Senate, it isn’t clear whether major parts of the legislation will make it past the Senate parliamentarian.

Republicans are using a special procedure that allows them to pass legislation with just 51 votes rather than the usual 60 votes.

But this process is limited to budget bills, so everything in the bill must be have a budgetary impact. The parliamentarian is considered the ultimate authority on how the Senate’s rules and procedures are interpreted and what can and cannot be included in the bill.

Senate Democrats released a list of provisions in the health care bill Friday that the parliamentarian has indicated would not pass muster. While the list is non-binding, it means that Democrats can raise objections to an array of provisions of the bill and may be able to strike them from the legislation.

For instance, the parliamentarian indicated that language that barring Planned Parenthood from receiving federal reimbursements for Medicaid services may not qualify for the 51-vote threshold. Conservatives have said that provision must remain in the legislation, but some moderate Republicans have opposed such a measure.

Senate Budget Committee spokesman Joe Brenckle dismissed the list as “guidance” on Friday and pointed out that the bill has been tweaked since the parliamentarian reviewed it.

“The parliamentarian has provided guidance on an earlier draft of the bill,

which will help inform action on the legislation going forward,” Brenckle said.

Contributing: David Jackson and Erin Kelly