The Senate is expected to vote Tuesday afternoon to begin debate on a Republican healthcare bill that would repeal portions of Obamacare, despite uncertainty over key details and whether enough Republican senators will vote for changes to the law.

GOP leadership is expected to call for a vote on a procedural motion that would start as much as 20 hours of debate on the House-passed healthcare bill, the American Health Care Act. If that motion gets the 50 votes it needs, assuming a tie-breaking vote by Vice President Mike Pence, the House bill would be stripped out and a new bill would be swapped in. It is not clear what that bill might be or if it could gain sufficient support.

Senate Majority Leader Mitch McConnell, R-Ky., announced Tuesday's vote on the Senate floor Monday. "I will vote yes on Motion to Proceed," he said. "I urge my colleagues to do the same."

Pence plans to be in the Senate to break a tie if there is one, Health and Human Services Secretary Tom Price told reporters Monday.

It's also so close that the office of Sen. John McCain, R-Ariz., said he planned to return Tuesday to continue working on the legislation and on other issues, even after his brain cancer diagnosis.

One of the bills being considered, the Better Care Reconciliation Act, would offer provisions to immediately replace parts of Obamacare, but it is controversial among centrists because it contains changes to Medicaid that would result in long-term spending cuts and is controversial among conservatives who say it does not go far enough in repealing Obamacare and driving down the cost of premiums.

The other bill, the Obamacare Repeal Reconciliation Act, would repeal Obamacare's taxes, mandates and spending but leave in place regulations on insurers and give lawmakers two years to come up with a new plan. Conservatives favor this route and point out that senators supported the same bill in 2015 when Barack Obama was president and vetoed it. Still, several lawmakers have said that they would change course this round. One GOP senator, Susan Collins of Maine, didn't support the 2015 bill and isn't expected to support it now.

Other Republicans, including Sens. Shelley Moore Capito of West Virginia and Lisa Murkowski of Alaska, have said that they do not support the plan, but they have not said if they will vote to begin debate.

Asked Monday what she would be voting on, Murkowski replied, "I'm told we are going to be finding that out. I would like to know more."

Those sentiments were echoed by Sen. Rand Paul, R-Ky., who favors the repeal-only bill.

"We have no earthly idea what we will be voting on," he said Monday on Fox News. "Last week they said we're going to vote on a clean repeal. I said hooray, that's what we promised. Now I'm told it may be the new Senate leadership bill, which is a big insurance bailout that I'm not for and doesn't repeal Obamacare."

David Popp, spokesman for McConnell, took issue on Twitter with characterizations that senators didn't know which bill they were voting on.

"It is a motion to proceed to the House bill," he wrote. "We've been crystal clear on that for weeks. And it's the rules."

Sen. John Barrasso, R-Wyo., echoed those sentiments.

"Anybody who has been in any legislative body knows absolutely you get a bill on the floor, people can look at it, write amendments ... if it was a real legislative process you don't know what the end product looks like until you've gone through all the amendments," he said.

Outside conservative groups have urged Republicans to fulfill their promise to repeal Obamacare, but some Republican governors are wary, particularly regarding changes that would be made to the Medicaid program.

Some of those groups met with McConnell Monday about the next steps.

One of the groups, Tea Party Patriots, said its supporters are calling senators asking for them to vote on the motion to proceed. After that, they want the straight repeal bill to go back to the House.

"Once the motion to proceed passes, the bill the Senate sends back to the House must be the 2015 bill to repeal Obamacare, which 51 Republican senators voted for 18 months ago," said Jenny Beth Martin, co-founder of Tea Party Patriot. "That is the absolute minimum our activists can support and is only the beginning."

Ohio Gov. John Kasich, a Republican, has been vocally opposed to the Republican plans on Obamacare, which would undo his state's Medicaid expansion. On Monday he released a statement saying that Americans "would come out on the losing end" if Republicans proceeded with the vote.

"Until Congress can step back from political gamesmanship and come together with a workable, bipartisan plan, it is a mistake for the Senate to proceed with a vote on Tuesday and force a one-sided deal that the American people are clearly against," he said. "Instead, they should make a commitment to bring Republicans and Democrats together to work openly on Obamacare's failings, which we all agree need to be fixed."

The bill is being passed through reconciliation, which requires at least 50 votes rather than the 60 typically needed to block a filibuster. Republicans have a slight majority in the Senate and cannot afford to lose more than two votes to move debate forward.

Several details about the bill are murky, including whether some anti-abortion provisions will be changed or removed, because the Senate parliamentarian recommended against them, saying they would not pass reconciliation rules. There also is question about how to replace Obamacare's individual mandate, which requires people to buy insurance or pay a fine and is intended to bring healthier customers into a risk pool to offset more expensive customers.

The bill is likely to change through amendments as well. One likely to be considered was introduced by Republican Sens. Lindsey Graham of South Carolina and Bill Cassidy of Louisiana, which would change much of Obamacare's funding into a block grant and allow states to craft their own healthcare plans. Cassidy has said he will vote to begin debate on healthcare legislation.

President Trump, speaking at the White House Monday, blamed Democrats and Republican holdouts for inaction on healthcare. Republicans are working only within their party on the bill because Democrats have said they are willing to fix Obamacare but will vote against the GOP plan that would repeal provisions of the law.

Trump aimed to make the case Monday that Republicans should fulfill their campaign "repeal and replace" promises, and met with Americans who said their lives had been hurt by the law, whether through higher premiums or losing access to certain doctors. He also issued a statement of administration policy that said he supports either healthcare bill Republicans choose.

"Any senator who votes against starting debate is telling America that you are fine with the Obamacare nightmare," Trump said. "The question for every senator, Democrat or Republican, is whether they will side with Obamacare's architects ... or with its forgotten victims."

Robert King contributed