(Sarah Parnass/The Washington Post)

Even if the House approves a GOP effort this week to repeal and replace key parts of the Affordable Care Act, the work of persuading the Senate to do the same is likely to be even harder.

Weeks of dealmaking in the House have not satisfied at least a dozen Republican senators who oppose the bill or lean against it. In a chamber where the GOP holds a slim 52-to-48 majority and where Democrats are united against the effort, just three Republican “no” votes would be enough to kill the measure.

In the House, which has been the epicenter of the health-care debate, GOP leaders postponed a planned Thursday vote on their bill amid struggles to meet the demands of conservative lawmakers. On Thursday night, President Trump delivered an ultimatum to House Republicans: Vote to approve the bill on Friday, or reject it and he would move on to other legislative priorities.

Some think the bill would go too far in undoing the ACA’s provisions, and with them the coverage on which many Americans have come to depend. Others say the effort falls short of their demand for a full repeal that would wipe out subsidies of insurance premiums and coverage mandates.

(Daron Taylor/The Washington Post)

In fact, a stark picture emerges from the statements of two dozen senators and staff members: Without significant changes, the measure stands virtually no chance of passing in the Senate. And such changes, in turn, could torpedo the effort when and if the House and Senate work out their final differences.

“I still have a lot of concerns,” said Sen. Lisa Murkowski (R-Alaska).

For many, a huge stumbling block is the pace.

Assuming the House approves the bill Friday, Senate leaders, led by Majority Leader Mitch McConnell (R-Ky.), have given themselves about a week to iron out disagreements and pass their own version of a bill to replace President Barack Obama’s signature domestic policy achievement.

“I am very suspicious of that being enough time, given what’s going on here,” said Sen. Lindsey O. Graham (R-S.C.).

Some lawmakers and staff members say the rush to the finish line could lead to mistakes in the legislation. Staff members on some key committees worry that skipping the usual vetting and debate would trade good policy for speed. Senators have expressed similar concerns.

“It’s more important to finally get health-care reform right than to get it fast,” Sen. Tom Cotton (R-Ark.) said in a statement this week. Cotton has been highly critical of the House measure.

How health care amendments appease GOP moderates and hard-liners

The biggest threat to the bill comes from frustrated Republican senators, particularly those who worry about provisions that, in 2020, would roll back the expansion of Medicaid under the Affordable Care Act to millions of working-class Americans.

Several GOP lawmakers from states that expanded Medicaid under the ACA say that would hurt the low-income and disabled people who obtain coverage though the program, particularly those battling drug addiction.

“I still have big, big concerns,” said Sen. Rob Portman (R-Ohio), whose state expanded Medicaid. He said he plans to introduce an amendment to address his concerns if the House doesn’t do so first.

“I’m not satisfied,” said Sen. Shelley Moore Capito (R-W.Va.), who has coordinated with Portman to try to tweak the bill. She added, “I’m worried about a lot of folks in West Virginia, of opioid and drug abuse issues, and the expanded Medicaid has helped with that.”

Many Senate Republicans have generally avoided speaking at length this week about the increasingly troubled House legislation, which has been thrust into doubt by opposition from conservative and centrist House Republicans. Senate GOP leaders have mostly avoided outlining an exact plan or strategy for how they plan to act before the House vote because of what one staff member called a fear of trying to commit to a moving target.

“We’re waiting to see what they send over,” said Sen. Roy Blunt (R-Mo.), a junior member of the Senate GOP leadership. “I doubt if there’s a lot of focus on this until . . . the House has actually passed a bill.”

If the legislation passes the House, it will go directly to the Senate, where McConnell has said he would fast-track it to a floor vote next week. The Senate plans to be consumed the following week with the GOP effort to confirm Supreme Court nominee Neil Gorsuch before leaving for a two-week recess.

Although senators would be allowed to offer amendments to the health-care overhaul, McConnell plans to skip the customary process of allowing Senate committees to debate the legislation. Republican leaders say the speedy push is necessary to avoid the threat of a blockade by Senate Democrats.

In addition to rolling back the Medicaid expansion beginning in 2020, the House bill would replace federal insurance subsidies in the ACA with age- and income-based tax credits. Those tax credits have generated controversy. Senate GOP leaders are expected to try to tweak them.

Senate Republican Conference Chairman John Thune (S.D.) is working on an amendment that would change the structure of the credits to better help older and lower-income Americans buy coverage.

“We have language on it with all the different thresholds and when it kicks in and how it varies among both age and income,” he said. Trump and his team are watching for Thune’s amendment with interest, said a White House official who spoke on the condition of anonymity.

The House bill also is under fire from a trio of conservative senators known for bucking party leaders: Sens. Ted Cruz (R-Tex.), Rand Paul (R-Ky.) and Mike Lee (R-Utah). They want to shred even more of the ACA than what the House bill targets. Lee and Paul have said they would vote against it in its current form, and Cruz has been very critical.

Senate Democrats, meanwhile, stand united against the effort. They have signaled that they will contest whether certain elements of the bill are permitted under Senate rules that would allow the measure to advance on a simple-majority vote only if its scope is limited to budget matters. Otherwise, a 60-vote victory — almost certainly unattainable in this case — would be necessary.

That means Democrats are likely to protest potential last-minute additions to the bill that conservatives want and that House lawmakers have been negotiating: ending the insurance mandates in the ACA that require plans to cover specific medical services such as mental health care, prescription drugs and preventive care.

A small number of conservatives, including Cruz and Lee, have argued that Republicans can go further within those rule constraints to attack the law, while some GOP leaders and Democrats have dismissed that notion. The emerging dispute could erupt next week.

That fear of commitment has spread to GOP members who represent states that accepted the Medicaid expansion. Normally chatty lawmakers, including Sens. Cory Gardner (R-Colo.) and Bill Cassidy (R-La.), waved off questions while Sen. Patrick J. Toomey (R-Pa.) avoided discussing the House legislation entirely. Others simply said they were far from sold on the House GOP plan.

Meanwhile, some Republicans expressed impatience about the rest of their agenda, which they can’t address until the health-care legislation is behind them.

“We are basically running out of time,” said Sen. John Cornyn (R-Tex.), McConnell’s top deputy. “We can’t get to tax reform or other things until we get this done.”

Senate Minority Leader Charles E. Schumer (D-N.Y.) and other top Democrats already plan to challenge whether major elements of the health-care bill, such as a ban on funding for Planned Parenthood and work requirements for those who use Medicaid, are legal under the strict budget rules. They hope to force GOP leaders to strike those provisions, which are critical elements of the law for many conservatives.

All of the forces working against the legislation have lowered the confidence of many Senate Republicans that their chamber can work out all of the kinks.

“We’ll see,” was all Sen. Jeff Flake (R-Ariz.) could muster.

Mike DeBonis, Juliet Eilperin and David Weigel contributed to this report.