WASHINGTON — Abortion flared up Wednesday as the latest hot-button issue to complicate passage of a bill to repeal and replace the Affordable Care Act, which Senate Republican leaders hope to unveil on Thursday and pass next week.

The repeal bill approved last month by the House would bar the use of federal tax credits to help purchase insurance plans that include coverage of abortion. But senators said that provision might have to be jettisoned from their version because of complicated Senate rules that Republicans are using to expedite passage of the bill and avoid a filibuster.

If that provision is dropped, a bill that has already elicited deep misgivings among moderate Republicans — and stiff resistance from Democrats, health care providers and patient advocacy groups — could also generate concern among abortion opponents, as well as conservative lawmakers.

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Further complicating the measure’s prospects, insurance companies, which took a leading role in the health care fights of 1993-94 and 2009-10 but have been conspicuously quiet this year, released a blistering letter objecting to Republican plans to remake Medicaid and cut its funding.

The changes being considered in Congress could “amount to a 25 percent shortfall in covering the actual cost of providing care to our nation’s neediest citizens,” the top executives of 10 insurance companies wrote. “These amounts spell deep cuts, not state flexibilities, in Medicaid.”

As senators struggle to develop a health care bill, their handiwork appears to be too moderate for some Senate conservatives and too conservative for some Senate moderates. The latest version, without the abortion-coverage prohibition and with steep Medicaid cuts, may prove unacceptable to some in both camps. To pass it, Senate leaders can afford to lose only two Republican votes of the 52 in the chamber.

Republican senators got a glimpse Wednesday of the highlights of the bill, which was drafted in secret by the majority leader, Senator Mitch McConnell of Kentucky, and top aides. White House officials were granted a formal briefing, which risked irking many senators who had yet to see the actual bill.

The House abortion provision has sweeping implications because many health plans subsidized under the Affordable Care Act include coverage for abortion services. The provision has encountered outspoken opposition from officials in states like Oregon, where most health plans on the public insurance exchange cover abortion.

But senators said the provision might have to be dropped for a more prosaic reason: It may not comply with the Senate rules that Republicans are using to speed the health care bill through the Senate.

The bill is scheduled to go to the Senate floor next week under these procedures, which limit debate and preclude a Democratic filibuster.

“It’s one of the problems we have to work with,” Senator Orrin G. Hatch, Republican of Utah and the chairman of the Finance Committee, said of the abortion issue. “We’re not quite sure how that’s going to be resolved.”

Mr. McConnell is determined to get a vote on the bill by the end of next week, before a break for the Fourth of July holiday, but he still does not have enough committed votes to ensure passage.

Senator Rand Paul, Republican of Kentucky, made clear on Wednesday that he was not on board with the Republican bill, which he said “sounds like Obamacare lite.”

“I’m still hoping we reach impasse, and we actually go back to the idea we originally started with, which was repealing Obamacare,” Mr. Paul said, adding, “I’m not for replacing Obamacare with Obamacare light.”

The House bill and the Senate version, like the Affordable Care Act, would provide tens of billions of dollars in tax credits to help people pay insurance premiums.

The federal government is expected to spend more than $30 billion this year on tax credits to help lower- and middle-income people pay premiums. The Senate bill would provide more assistance to lower-income people than the House bill, which bases tax credits on a person’s age.

The Senate bill would also repeal most of the taxes imposed by the Affordable Care Act. It would delay the effective date of a tax on high-cost employer-sponsored health coverage, but Republicans plan to offer an amendment next week to eliminate this “Cadillac tax,” which is opposed by labor unions and employers.

Senators Thom Tillis of North Carolina and Susan Collins of Maine, both Republicans, said they understood that the House restrictions on the use of tax credits for insurance covering abortion had encountered parliamentary problems.

“What I heard earlier from the parliamentarian is they didn’t think it would pass” muster under Senate rules, Mr. Tillis said.

Mr. Tillis and Ms. Collins said they understood that Senate Republican leaders were hoping to devise some kind of workaround to address concerns of anti-abortion lawmakers. But it was not clear whether those anti-abortion lawmakers would be satisfied with such a plan, which could involve separate legislation.

Republicans have been promising to repeal the health law ever since it was signed by President Barack Obama in 2010. On Wednesday, in the final hours before the Senate repeal bill was to be unveiled, members of Congress, consumer groups and health care executives engaged in frenetic advocacy in hopes of shaping the bill.

Women’s groups and at least two moderate Republicans, Ms. Collins and Senator Lisa Murkowski of Alaska, continued to object to a provision of Mr. McConnell’s bill that would cut off funds for Planned Parenthood.

The leaders of 10 insurance companies told Mr. McConnell that proposed caps on federal Medicaid spending would cause “an enormous cost shift to the states,” forcing them to raise taxes, reduce benefits, cut payments to health care providers or eliminate coverage for some beneficiaries. Among those who signed the letter were top executives from AmeriHealth Caritas, Molina Healthcare, Blue Shield of California and Healthfirst, in New York.

But Senator John Kennedy, Republican of Louisiana, said the Medicaid provisions were one of the bill’s chief attractions for him.

“In my state,” Mr. Kennedy said, “we are now spending 47 percent of our budget on Medicaid. That’s up from 23 percent in 2008. It’s crowding out money for universities and roads and public safety and coastal restoration, and it just keeps climbing.”

Even senators who might support the legislation said they did not want to be rushed.

Asked how he felt about the prospect of voting for a bill a week after its release, Senator John McCain, Republican of Arizona, said, “I’d feel terrible about it.”

Senator Ron Johnson, Republican of Wisconsin, said, “I need the information, I need to hear from constituents, and that’s going to take some time.”

Debate on the Senate bill will be shaped by an analysis from the Congressional Budget Office, which will estimate the impact on federal spending and the number of people without health insurance. Under the House bill, the office said, the number of uninsured would be 23 million higher than under the Affordable Care Act in 2026. And for some older Americans and sick people, it said, premiums and out-of-pocket costs could be significantly higher.