(Bloomberg) —President Donald Trump called on Senate Republicans to start the process of repealing the Affordable Care Act, warning that anyone who votes not to take up the bill Tuesday is saying they are “fine with the Obamacare nightmare.”

“For Senate Republicans this is their chance to keep their promise,” Trump said at the White House on Monday. “There has been enough talk and no action. Now is the time for action.”

Senate GOP leaders still haven’t decided which health-care proposal they’ll ask members to vote on, in what has become a series of all-out efforts to get their own members on board.

Majority Leader Mitch McConnell said the Senate will vote Tuesday on whether to begin debate.

If the Senate agrees to take up a bill, members will have a chance to vote on whatever proposals and amendments they want to offer, said second-ranking Republican John Cornyn of Texas. McConnell said among them will be the 2015 Obamacare-repeal bill with a two-year delay that was vetoed by then-President Barack Obama. Trump would sign such a bill, the majority leader said.

Republicans are struggling for a way to fulfill years of promises by party leaders to get rid of Obamacare. They’ve been unable to find a replacement plan that can attract at least 50 votes in the Senate, where the party has a 52-48 majority. McConnell is trying to coax support from moderate and conservative Republicans who have raised objections amid unified Democratic opposition.

Vice President Mike Pence will preside over the Senate in case his vote is needed to break a tie, Health and Human Services Secretary Tom Price told reporters. Trump’s budget office issued a statement urging senators to agree to begin debate.

McCain’s Brain Cancer

Several Republicans have already said they oppose repealing Obamacare without a replacement, including Senator Susan Collins of Maine.

“It’s my understanding that the leadership is going to meet tonight and decide” what to do this week, Collins said in an interview.

One difficulty for the GOP is Arizona Senator John McCain’s brain-cancer diagnosis last week. He has said he wants to return to Washington as soon as he can, though he hasn’t given a timetable. Cornyn said he hopes McCain will be able to travel to Washington to vote Tuesday, but isn’t sure yet that he will.

Senator Pat Roberts of Kansas, a GOP member of the Health committee, said senators are discussing revisions to McConnell’s plan to replace Obamacare, which collapsed for lack of support last week. The majority leader then said he would seek to bring a simple repeal of Obamacare to the floor early this week, but that proposal also fell apart amid opposition from Collins and others.

‘Not the End’

“It’s still fluid,” said Roberts, who added that he wants to support whatever plan emerges because he opposes leaving Obamacare in place.

Cornyn said the GOP won’t give up on replacing Obamacare if the Senate can’t pass it this week. “If for some reason we aren’t able to muster the votes tomorrow, which I’m increasingly optimistic we will, it’s not the end of it,” he said.

Senator John Barrasso of Wyoming, a member of the GOP leadership team, said Sunday on CBS’s “Face the Nation” that the plan is to hold a procedural vote on whether to bring the House-passed health bill to the floor.

Then, senators could “amend it in various ways and lots of members have different ideas on how it should be best amended,” Barrasso said. “Until the vote is actually on the floor of the Senate, some people may not tell you what they’re actually going to do.”

Susan Collins, West Virginia Senator Shelley Moore Capito and Alaska Senator Lisa Murkowski said last week they would oppose a repeal-only plan. Murkowski said Monday she was undecided on whether to vote to begin debate. “I would like to know more, as I’m sure all of you would too,” she said.

On the other end of the GOP’s ideological spectrum, Senator  Rand Paul of Kentucky wants legislation that more fully repeals the Affordable Care Act.

Roberts said his main concern is the effect of spending cuts on rural hospitals that get Medicaid funds. “Under the current system, they’re just not going to be able to make it,” Roberts said. “We’ve just got to figure out a way to make that work better.”

He said he was in touch over the weekend with Seema Verma, the Trump administration’s head of Medicare and Medicaid, and that her proposal to make up some funding with non-Medicaid money has potential.

It will be hard to get people who have taken hard stands against McConnell’s plan or portions of it to change their positions, Roberts said.

“It’s awfully difficult when people climb the tree and get out on a limb and say, ‘I’m going to vote no,”’ the Kansas senator said. “For them to skinny back down that tree, that’s tough. And they have to have some very good reasons as to why that’s the case.”

‘Disastrous ObamaCare’

Trump has alternated between calling on lawmakers to let Obamacare die and insisting that Republicans come up with a replacement plan. Late Sunday, he wrote on Twitter: “If Republicans don’t Repeal and Replace the disastrous ObamaCare, the repercussions will be far greater than any of them understand!”

Efforts to resurrect McConnell’s Obamacare replacement suffered a further blow on Friday when the Senate parliamentarian issued a preliminary finding that key parts of the proposal don’t qualify for a fast-track procedure being used by the GOP. Those parts would require 60 votes rather than the simple majority the GOP seeks to use, Parliamentarian Elizabeth MacDonough said.

Among them are provisions that would defund Planned Parenthood for a year, prevent tax credits from being used to buy insurance policies that cover abortion, and encourage people to have continuous insurance by barring those without coverage for at least two months from buying new insurance for half a year.