Updated on March 7, 2017 at 7:29 p.m. ET

A Republican alternative to Obamacare years in the making was on the verge of unraveling just a day after its introduction, as conservative lawmakers, advocacy groups, and industry leaders denounced it as an insufficient answer to the nation’s healthcare challenges.

Party leaders in Congress and in the Trump administration held up the American Healthcare Act as the GOP’s long-awaited deliverance on its promise to repeal and replace the Affordable Care Act with “patient-centered” market reforms, hoping to pass the bill through the House and Senate within a matter of weeks. But by Tuesday evening, it wasn’t clear whether the legislation would ever make it to a floor vote, and it was far easier to find lawmakers and organizations who were against the bill than it was to find those in favor of it.

Republicans in Congress were increasingly looking to President Trump to help lift their flagging legislation.

Despite a full-throated endorsement from the president and personal lobbying from Vice President Mike Pence, however, conservatives urged the House leadership to set aside the new bill and instead vote on a straight-forward repeal of Obamacare. They criticized the proposal as “Obamacare Lite,” arguing that it created a new entitlement program in the form of tax credits for health insurance and maintained the current law’s Medicaid expansion and some of its tax increases. “We are united on repeal, but we are divided on replacement,” Senator Rand Paul of Kentucky declared at an afternoon press conference. He stood alongside several members of the conservative House Freedom Caucus and with Senator Mike Lee of Utah, who said the House bill was “a step in the wrong direction and as much as anything a missed opportunity.”

The proposal’s supporters, however, were publicly unfazed. “Doing big things is never easy, but we have made a promise, and we are going to keep that promise,” House Speaker Paul Ryan said. He defended the bill as a dismantling of Obamacare and confidently predicted it would pass over conservative objections. “We will have 218 [votes] when this thing comes to the floor, I can guarantee you that,” Ryan said, referring to the threshold it would need to secure a majority in the 435-member chamber. (The actual threshold may be lower due to the five current vacancies.) At the White House, Trump was similarly upbeat as he met with the House GOP’s team of deputy whips responsible for shepherding the legislation to passage. “It’s a great bill,” the president said. “I really believe we’re going to have tremendous support. I’m already seeing it, not only from people in this room but from everybody.”

It wasn’t clear if Trump was aware of the forces arraying against the proposal outside the White House. Democrats are expected to vote against the bill without exception. In addition to conservatives in Congress, Republican Governors John Kasich of Ohio, Bruce Rauner of Illinois, and Paul LePage of Maine were among those who criticized the measure. The conservative advocacy groups Heritage Action, FreedomWorks, the Club for Growth, and the Koch Brothers-backed Americans for Prosperity all denounced the bill. The American Hospital Association issued a letter of opposition from its president and CEO, Richard Pollack. In the Senate, a member of the Republican leadership, Roy Blunt of Missouri, voiced doubts that the bill could secure a majority in either chamber.

Whether the initial opposition turns out to be a bump on the long road to passage or a fatal blow remains to be seen. The legislative process on Capitol Hill is almost always a roller-coaster, and that was never more true than when Democrats passed the Affordable Care Act seven years ago. The law took more than a year to get through Congress, and it appeared dead after Democrats lost their filibuster-proof majority in the Senate early in 2010. On Tuesday, Republicans vowed to press ahead, and both the House Energy and Commerce and Ways and Means Committee will go forward with plans to mark up their legislation on Wednesday.

Still, the intensity of the opposition on Tuesday was striking, and it appeared—at first glance—to explain why it took so many years for Republican leaders to embrace a detailed legislative alternative to Obamacare. The attacks arrived even before the Congressional Budget Office had a chance to fully assess the bill’s impact on the deficit and insurance coverage, which could become another flashpoint. Republicans were already anticipating, and preparing to rebut, a CBO projection that their bill would lead to an increase in the number of people without insurance.

At points during the day, whether the new proposal was even open to further negotiation was a matter of debate. While the White House embraced the bill as its own, Tom Price, the secretary of health and human services, repeatedly declined to say whether the administration supported every aspect of it. The proposal, he said, was “a work in progress.” Later in the afternoon, members of the Freedom Caucus said they had been told directly by Pence that party leaders would be willing to negotiate changes.

But by the evening, Trump began whipping the vote on Twitter. His first target was Paul. “I feel sure,” the president wrote, “that my friend @RandPaul will come along with the new and great health care program because he knows Obamacare is a disaster!”

The effort to pass the healthcare bill will be a defining battle for Ryan as well as Trump’s first major legislative test. Republicans were leaning on the president’s support and hoping that his popularity among the rank-and-file would bring conservatives onboard. After just one day, however, their chances of success seem increasingly in doubt.