WASHINGTON — Senate Republican leaders unveiled a fresh proposal to repeal and replace the Affordable Care Act, revising their bill to help hold down insurance costs for consumers while keeping a pair of taxes on high-income people that they had planned to eliminate.

But the measure was immediately imperiled when two Republican senators, moderate Susan Collins of Maine and conservative Rand Paul of Kentucky, announced they were not swayed — even on a procedural motion to take up the bill next week, a motion to proceed.

One more defection would doom the bill and jeopardize the Republicans’ seven-year-old quest to dismantle the health law that is a pillar of President Barack Obama’s legacy. In a sign that more could follow, two other Republican senators, Lindsey Graham of South Carolina and Bill Cassidy of Louisiana, unveiled their own alternative plan, just minutes before Senate leaders offered their latest.

Senator Shelley Moore Capito, Republican of West Virginia, expressed “serious concerns about the Medicaid provisions” in the latest draft, although she did not reject it.

With the revised bill, the majority leader, Mitch McConnell, Republican of Kentucky, had hoped to win the 50 votes he needs to win Senate passage. But the changes may not have been enough to bridge the vast divide that has opened between the Senate’s most conservative Republicans, who had vowed to destroy the Affordable Care Act “root and branch,” and its moderate Republicans, who worry that deep cuts to Medicaid would leave too many in their states without health care.

Republicans said the revised bill would provide roughly $70 billion in additional funds that states could use to help reduce premiums, hold down out-of-pocket costs and otherwise make health care more affordable. The bill already included more than $100 billion for such purposes.

But the new bill, like earlier versions, would still convert Medicaid from an open-ended entitlement to a system of fixed payments to states. In the event of a public health emergency, state Medicaid spending in a particular part of a state would not be counted toward the spending limits, known as per capita caps, a concession to moderate Republicans but perhaps not enough to get the 50 votes needed for passage.

Overall, the new version of the bill made broad concessions to conservative Republicans who had maintained that the initial draft left too much of the Affordable Care Act in place. Mr. McConnell then backfilled the bill with money intended to placate moderates. That jury-rigging of the bill left neither side completely satisfied.

For instance, in a departure from current law, the bill would allow insurers, under certain conditions, to offer health plans that did not comply with standards in the Affordable Care Act. Under that law, insurers sell regulated health plans through a public insurance exchange in each state.

But health care experts worried that such a change would send healthy consumers to low-cost, basic health plans, leaving sick and older consumers to buy more comprehensive health policies at much higher prices….