States have two other options: They can choose to reject federal funding entirely and give up on the whole A.C.A. project or seek “innovation waivers” to devise their own federally funded program.

Senate Democrats should engage with the Cassidy-Collins sponsors and other Republican senators who have expressed qualms about the Ryan proposal. Many of those senators are in states that embraced the A.C.A. Medicaid expansion, collectively slashing the ranks of their uninsured by 45 percent.

But they generally call for delaying Medicaid’s phaseout, not stopping it. Even the Republican governors in states that embraced the A.C.A.’s Medicaid expansion, posing as champions of its beneficiaries, floated an alternative plan that left expansion states with a Catch-22: either accept per capita caps on the federal Medicaid contribution, or give up enhanced federal funding for the expansion’s beneficiaries. Left to their own devices, the Senate moderates are likely to prove equally limp champions of the expansion, or of the federal commitment to making health insurance affordable for all Americans.

This is why it’s urgent for Democrats to act now. If a quorum of three or more of this demurring dozen is to block these destructive changes — and defy the president — they need an alternative path. Democrats can provide it by working with Senators Cassidy and Collins, and others who wish to devote Obamacare-level resources to extending coverage.

Of course they will have to overcome some natural aversion. Allowing states to junk the A.C.A.’s coverage rules, channel subsidies through health savings accounts (which generally work only for people with significant resources), distribute a reduced subsidy pot to wealthier individuals or opt out of subsidizing coverage may strike many as a poison pill.

But Republican governors and legislatures that wish to do these things can do them right now, with the exception of opting out of subsidizing coverage entirely. The A.C.A. allows states to seek innovation waivers from the Department of Health and Human Services to remake their marketplaces, proposing alternatives to the law’s coverage rules, subsidy formulas and employer and individual mandates. The state must show that the alternative scheme will cover as many people as Obamacare would, as comprehensively and at comparable cost. With Tom Price as the secretary of Health and Human Services, conservative state governments could doubtless win approval for any plan they proposed.