Walker’s announcement came a day before Donald Trump addresses a joint session of Congress, and on the same day that the president promised to offer “something special” on his health-care overhaul efforts.

But the comments from Walker are a potentially serious blow for the plan by Republican leaders to push through a bill in the coming weeks to repeal and replace the polarizing health-care law.

Several conservatives have already voiced concerns about the strategy not going far enough to dismantle Obamacare, while other Republicans have been battered by constituents in town halls over the likelihood that the GOP’s replacement would insure fewer Americans.

Related: Former GOP House speaker thinks ACA system will stay

The support of Walker, a North Carolina Republican, is important, given that the plan cannot be passed in the 435-seat House without almost unified Republican support, since no Democratic backing is likely.

Outdated draft

Walker’s statement refers to a two-week old draft of the Republican plan, leaked last week, which he said "risks continuing major Obamacare entitlement expansions and delays any reforms."

Related: Trump courts governors, health insurer CEOs

House Republicans are set to meet behind closed doors on Tuesday morning, where they are expected to discuss the path ahead. Many leaders are counting on the strong pressure from outside conservative groups pushing for a repeal to keep Republicans in line behind their plan, but the path forward remains difficult.

Trump’s administration has promised elements of a replacement plan by mid-March. But the president’s public statements have sometimes been at odds with GOP lawmakers. Trump has promised better, less expensive care for all without cutting entitlement programs, such as Medicare, while Republicans in Congress have admitted that their plan is likely to cover fewer of the 20 million people who gained coverage under the Affordable Care Act.

‘Republican stamp’

Walker’s statement serves only to underscore that continued lack of cohesion.

“The bill contains what increasingly appears to be a new health-insurance entitlement with a Republican stamp on it," said Walker, who had already been raising concerns over the proposed inclusion of refundable tax credits in the plan.

House Republican leadership favors that idea, but Walker said in a recent interview that conservatives believe "we’ve got to make sure we are not adding additional entitlement giveaways."

House Ways and Means Chair Kevin Brady said Monday that the leaked legislative language in question is "pretty outdated" in its numbers and that a lot of changes have been made.

But Brady declined to offer details on the new language being considered, making it difficult to know whether the new proposals are any more palatable to conservatives. Republican leaders haven’t abandoned the refundable tax credit, viewing it as necessary to provide assistance to low-income Americans who have little or no net tax bill.

‘Rubik’s cube’

"This is a continuous loop. We’re updating our drafts on almost a daily basis," Brady said. "So it is a little like turning a Rubik’s Cube. We continue to bring specifications to them, they give us feedback, we make adjustments, they give us feedback."

Asked what he’s learned from the process of trying to overhaul the health-care system, Brady said: "This is enormously complicated work."

A wild card in the debate is Trump, who is expected to bring up the issue during his joint speech to Congress on Tuesday evening. Trump has previously said he wants to insure everybody but has refrained from discussing policy details. He said earlier on Monday that he’ll soon release a "fantastic plan."

The president also seemed taken aback by the complexity of the endeavor.

"Nobody knew that health care could be so complicated," he told a meeting of the National Governors Association.

Democratic unity

Walker’s announcement comes as Democrats in both chambers are growing increasingly confident that their strategy of uniting against a Republican alternative is working and that the inability of Republicans to coalesce around a replacement plan will doom the effort.

“Republicans have not been able to pick off a single Democrat to support their plans,” Senate Minority Leader Chuck Schumer told reporters Monday. “We have shown a united front, while Republican internecine warfare has been on full display.”

“I predict the discord in their party will grow as Republicans turn to Washington from this last week of angry town halls,” Schumer added. “I believe the odds are very high that we will keep the ACA and it will not be repealed.”

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