WASHINGTON — Senate Republican leaders on Thursday unveiled a fresh proposal to repeal and replace the Affordable Care Act, revising their bill to help hold down insurance costs for consumers while allowing insurers to sell new low-cost, stripped down policies.

Those changes and others, including a decision to keep a pair of taxes on high-income people and to expand the use of tax-favored health savings accounts, were intended to bridge a vast gap between the Senate’s most conservative Republicans, who want less regulation of health insurance, and moderate Republicans concerned about people who would be left uninsured.

But Republican leaders will have to battle for votes ahead of a final showdown they hope will come next week. Two Republican senators, Susan Collins of Maine, a moderate, and Rand Paul of Kentucky, a conservative, said they were not swayed — even on a procedural motion to take up the bill for debate.

Several others, from both sides of the party’s ideological spectrum, expressed misgivings.

Senator Mike Lee, Republican of Utah and a strong conservative, said, “The new Senate health care bill is substantially different from the version released last month, and it is unclear to me whether it has improved.”