Senate Majority Leader Mitch McConnell (R-Ky.) on Capitol Hill. (Andrew Harrer/Bloomberg News)

Senate Republicans plan to return from a week-long holiday recess with a partial draft of a bill to reshape the nation’s health-care laws. But significant and persistent divisions in their ranks have cast the prospect of passing legislation into serious doubt after weeks of discussion.

Majority Leader Mitch McConnell (R-Ky.) and his top deputies have bluntly voiced uncertainty about how they will round up the 50 votes needed to pass a bill. And they have declined to give details about their emerging measure, which is expected to look different from a House-passed bill that has drawn criticism from some Republicans who say it went too far in undoing the Affordable Care Act, also known as Obamacare.

“I think people’s staff will start putting together some language we can look at when we return,” said Sen. John Cornyn (Tex.), the second-ranking Republican. But he said, “There is no final agreement yet.”

Others were less diplomatic about the disharmony in the effort to fulfill one of the GOP’s signature campaign promises. The biggest issues yet to be resolved, according to Republican senators, include how expansively the federal government should fund Medicaid, whether to enable states to avoid key ACA regulations and how to structure tax credits to replace federal insurance subsides offered under the current law.

“We’re having meetings, discussions about it to the point of utter boredom every lunch that we have,” said Sen. John McCain (R-Ariz.). “So I think they’re trying to build consensus, and there isn’t consensus yet.”

(Peter Stevenson/The Washington Post)

McConnell and the top Republicans on three committees are expected to work with staff to take the lead on crafting the measure, GOP aides said.

But what their final product will look like and when it will be ready for a vote were up in the air as senators dispersed to their home states for the one-week Memorial Day recess Thursday. With a 52-to-48 advantage over Democrats, Republicans can afford only two defections. McConnell told Reuters this week that “I don’t know how” to get to 50 votes. And Senate Republican Conference Chairman John Thune (S.D.) offered a less than confident “I hope so,” when asked if Republicans can get to 50.

“We need to establish firmly what our goals are,” said Sen. Bill Cassidy (R-La.), who has introduced a health-care proposal with Sen. Susan Collins (R-Maine). “It is a little nebulous.”

Despite McConnell’s public push to distinguish the Senate health-care effort from the secretive House process by focusing recent Republican lunches on health care and meeting with a smaller group of senators to discuss it, some critics say he is repeating the lower chamber’s mistake of crafting the bill too privately.

“It’s one way to go forward,” Collins said. “I think it’s always helpful to have some public hearings and bring in some experts.”

Cassidy was among the Republican senators who attended a Senate GOP health-care working group meeting Thursday. The ideologically diverse group of senators, which meets twice a week and has grown to more than 13, was designed as a testing ground for ideas and a way to educate lawmakers about the intricacies of writing a health-care bill.

“We have not seen senators coming out to the press and drawing lines in the sand and saying if A, B and C aren’t in the bill then I’m out,” said Sen. Ted Cruz (R-Tex.), one of the group’s founders. “You have a whole bunch of senators doing that. That breaks the process down quickly.”

Even without such pronouncements, however, the wide gaps in policy and politics among Republican senators have become apparent.

One area of disagreement is the question of whether to allow states to opt out of some ACA regulations, such as the requirement that insurers do not charge people with preexisting medical conditions more than healthy individuals. The House bill allows this.

Cruz said he wants to give states more flexibility in lifting some ACA requirements on insurers to give people access to cheaper, leaner plans. But Cassidy expressed concerns that state waivers would translate into fewer benefits and trouble for those with preexisting conditions.

The House bill has come under renewed scrutiny following a nonpartisan Congressional Budget Office analysis released this week that showed it would leave 23 million more Americans uninsured by 2026 than under current law and would force some less healthy individuals to pay more for coverage.

Even though Senate Republicans are writing their own bill, the House bill is still relevant to their task. A key question they face is whether all of the elements of the House measure are compatible with the procedural maneuver known as budget reconciliation, which they have opted to use to avoid having to clear a 60-vote threshold. Instead, they will need only a simple majority.

Democrats have raised some doubts about the House bill’s compatibility with reconciliation rules. The nonpartisan Senate parliamentarian will review the bill and is likely to rule that parts of it, such as its repeal of some ACA insurer regulations, for example, must be stripped because they don’t affect federal spending directly enough.

The review process will also inform senators about what they can fit into their own measure. So will the projected deficit reduction of the House bill: $119 billion over a decade.

Medicaid and tax credits are the two other contentious topics with which senators are wrestling. Some Republican senators from states that accepted the expansion of Medicaid under the ACA have been pushing for a lighter touch on the program than the House bill would deliver and more generous tax credits so fewer people would lose coverage.

But a trio of conservative senators, Cruz, Mike Lee (R-Utah) and Rand Paul (R-Ky.), have indicated a discomfort with the House bill for not pulling back harder on the ACA.

The House bill would cut $880 billion from Medicaid during the next decade, which provides coverage for low-income Americans and helps pay for long-term care for people with disabilities and seniors. It would also phase out the Obamacare-era expansion of the program in 2020 and change the way it disburses coverage payments.

Sen. Rob Portman (R-Ohio), whose state expanded Medicaid, said this week that he continues to push for a more gradual “glide path” rollback, as well as ensuring protections for people battling opioid addiction.

“I believe there is a way to provide for a mechanism to help people who are currently getting coverage on Medicaid expansion,” Portman said.

“How they handle Medicaid certainly is an issue for me,” said Sen. Dean Heller (R-Nev.), whose state also expanded Medicaid.

Democrats have united against the Republican push to repeal and replace parts of the ACA. McConnell is not counting on winning a single crossover vote. If a final vote is a 50-50 tie, Vice President Pence would have the ability to break it in Republicans’ favor.

For some Republicans, the intense partisanship is problematic.

“A Republican plan to replace a Democratic plan is not what the country needs,” said Sen. Lindsey O. Graham (R-S.C.). “We need a bipartisan plan.”

Even if Republican senators come up with legislation capable of winning enough support from their own ranks, they must obtain yet another CBO score and a thumbs-up from the parliamentarian before holding a vote on it.

All of that would take several weeks, at a minimum, making it a challenge for the Senate to vote on anything before lawmakers leave town for August recess.

Asked whether there might be a health-care vote before that time, McCain responded: “Damned if I know.”

“But I know this,” he added. “Time is not on our side.”

Correction: An earlier version of this story misstated how the House bill would affect ACA regulations. It has been corrected.

Ed O’Keefe contributed to this report.