After six years of legislative dry runs, the battle over repealing the Affordable Care Act is suddenly getting very real.

Republicans in the Senate on Wednesday voted to formally set in motion a complicated, multistep process that—they hope—will result in President Donald Trump signing a law in his first weeks in office that would scrap the key parts of President Obama’s signature domestic policy achievement. On both sides of the Capitol, the parties began mobilizing for what will likely be the first pitched policy fight of the Trump presidency.

As Obama made a personal appeal to Democrats to fight to save his legacy, Vice President-elect Mike Pence rallied Republicans behind repeal and left little doubt that it would be a top priority for the new administration. “The first order of business is to repeal and replace Obamacare,” Pence declared to reporters in separate press conferences after he met first with House Republicans and then with their counterparts in the Senate.

Yet despite the confident pose of party leaders, questions about the GOP strategy lingered: Would they really go through with repeal? And more critically, can they muster the votes needed to undo the health law before they coalesce around a plan to replace it?

In the Senate’s first action of the new Congress, Senator Rand Paul of Kentucky cast a surprising vote against a procedural motion to begin debating a budget bill designed to allow the Senate to repeal Obamacare with a simple majority of 51 votes rather than the usual 60 needed to break a filibuster. Paul cited the deficit-busting nature of the budget, but his opposition meant that because of the GOP’s narrower 52-48 seat majority, the resolution advanced by just a single vote. (Republicans will gain a bit more breathing room once Pence becomes the tie-breaking vote as vice president.)

And in the House, lawmakers said Republicans were not as unified around an Obamacare strategy as their leaders’ optimism would suggest. “There’s not a consensus on how we should proceed,” Representative Charlie Dent, a centrist Republican from Pennsylvania, told me in a phone interview.

The divide is not about repeal itself: Virtually all Republicans in Congress support scrapping the Affordable Care Act, just as every single one serving in 2010 voted against creating the law in the first place. But Dent, Paul, and a number of others are questioning the way party leaders plan to go about repeal.

The current “repeal and delay” approach calls for passing the repeal in the next month or so but setting the date of enactment anywhere from two to four years from now. That would buy the party time to draft and pass a replacement while providing for, in Pence’s words, “an orderly transition” out of Obamacare. At the same time, Pence told Republicans that Trump would sign executive orders in his first days in office related to the health law. He did not specify what the orders would entail, and a transition spokesman declined to elaborate.

“We want to make sure that we don’t pull the rug out from anybody during that transition. That’s the point we’re all trying to make,” House Speaker Paul Ryan told reporters after the Pence meeting.

Yet that “transition period” is exactly what’s worrying Democrats, health insurers, doctors, hospitals and other health industry stakeholders—virtually all of whom have warned Republicans that repealing the law without simultaneously replacing it could destabilize the insurance market and threaten coverage for millions. And that potential gap between repeal and replace is increasingly worrying Republicans like Dent as well.

“This is potentially a perilous situation,” the congressman said. “If we repeal and delay and hope to replace, we’re setting ourselves up for a real problem.” Dent said that after Pence left the House Republican meeting, he voiced those worries in a separate gathering of senior GOP lawmakers, and several shared his opinion. “I am concerned about a strategy of repeal and delay and hope to replace sometime down the road,” Dent added. “That’s simply hoping for the best, but we should be planning for the worst.”

The debate over how to replace Obamacare, he said, should occur “before we vote on the repeal measure rather than after.”

Representative Tom Cole of Oklahoma, a veteran party strategist who is close to the GOP leadership, told me that a number of other “very solid, very thoughtful, very influential” members had raised similar concerns. But he said that with Republicans in the House enjoying a comfortable, 23-seat majority, he did not believe the wavering lawmakers would be enough to derail repeal. “The conference is so wedded to the repeal idea after six years of fighting this thing,” Cole said. “I think that train is leaving the station regardless.”

At the most senior levels, the debate over repeal is beginning to resemble the debate over passage six years ago: Leaders in both parties appear supremely confident that they have the public on their side. With a boost from Obama, Democrats said they were united against repeal and warned Republicans that they would bear the political blame if voters lost their insurance or faced higher costs and fewer protections and benefits as a result of repeal. The GOP, said Senator Charles Schumer of New York, the Democratic leader, “are like the dog that caught the bus.”

“They can’t keep all the things that the American people like about the ACA and get rid of the rest without throwing our entire healthcare system—not just those on ACA, but those on private insurance—into chaos,” he said. Democrats and outside advocacy groups have already begun targeting swing states and districts with ads and grassroots pressure, aimed at swinging the three Republican senators they’ll need to block repeal.

Schumer added a new twist on Wednesday: a Trump-style slogan. “The Republican plan to cut health care,” he said, “wouldn’t make America great again, it would make America sick again, and lead to chaos instead of affordable care.”

Despite the misgivings in their rank-and-file, leading Republicans were equally self-assured they could win the political battle by reminding the public that they were merely fixing a mess that Democrats created. The watchword from Ryan and his colleagues was “relief” from an onerous, costly law. “The American people want us to start over and repeal Obamacare,” Pence declared. As for the fallout, he said: “The simple fact is that the American people know who owns Obamacare. It is the first half of that title.” Polls have shown that while support for the law is split, most Americans oppose a full repeal.

If anyone in the top echelon of the Republican Party betrayed concern about the emerging strategy in Congress, it wasn’t the speaker, the vice president-elect, or the Senate majority leader, Mitch McConnell. Instead, it was Trump himself. In a series of morning tweets, the president-elect warned Republicans to “be careful” in aggressively scrapping a law that, he said, will “fall of its weight.”

Exactly what Trump meant wasn’t immediately clear. Cole surmised that he was attempting to triangulate politically, much as he’d done on Tuesday when he publicly questioned the timing of a GOP move to gut an independent congressional ethics office. Following that tweet and a flurry of angry constituent calls, Republicans backed off.

Could Trump have a similar impact on the GOP’s repeal-and-delay strategy? Cole wasn’t sure, but he said the president-elect had already demonstrated an impressive amount of pull with Republicans in Congress—more, the congressman said, than he either Ryan or his predecessor, John Boehner, ever had. “In the end,” Cole told me, “we’re not going to do something he doesn’t want us to do.”