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.

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 this week. “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…