A member of the audience holds up a placard as Sen. Bernie Sanders (I-Vt.) discusses Medicare for All legislation on Capitol Hill on Sept. 13. (Jim Watson/AFP/Getty Images)

One year ago, with the flick of his thumb, Sen. John McCain (R-Ariz.) foiled the Republican Party’s quest to undo the Affordable Care Act and fulfill a seven-year promise to remake the health-care system.

Now, three months from the midterm elections, health care remains a gaping political vulnerability for the GOP. Although Republicans have been unable to produce an alternative to the law, they have succeed in undoing key provisions that critics say are leading to rising premiums for individual buyers of health insurance.

The Trump administration has acted unilaterally to undo aspects of the Obama-era law through executive powers, cutting insurer subsidy payments, slashing advertising and targeting the legislation in courts. The steps have pleased conservatives but have undermined an increasingly popular law that tens of millions of voters rely on.

Democrats consider health care — more than any other issue — their best chance to persuade swing voters in key races nationwide. The party’s candidates and political committees are already on air with ads targeting Republicans who backed the “repeal and replace” effort, and they expect to spend tens of millions of dollars attacking the GOP on health care over the next 100 days.

“It’s effective everywhere,” said Charlie Kelly, executive director of the House Majority PAC. “This is an issue that’s been Number One across the board in every election. I don’t see that changing, and it’s something we’re going to be talking about from now until Election Day.”

Recent polls have shown that health care is one of the top issues motivating voters, alongside jobs and the economy. A Washington Post-Schar School poll in July found that Democrats have a clear advantage among those voters who cite health care as their most important issue, and a Pew Foundation poll in June found that voters trust Democrats over Republicans on the issue by a 16-point margin.

Kelly’s group just this week began running a new round of ads targeting Republican House incumbents for their support of the failed GOP health-care bill. The ads highlight an “age tax” — a provision that would have let insurers to charge older policyholders up to five times as much as younger ones, reversing an ACA regulation that allows no more than a 3 to 1 ratio.

Many Republican lawmakers and strategists interviewed this week conceded that GOP candidates are broadly vulnerable on the issue. Highlighting the concern, the House this week voted on several bills aimed at lowering health-care costs, including a measure that would expand untaxed health savings accounts and allow for the sale of cheaper, less comprehensive plans, as well as another bill that would repeal an ACA tax on medical devices.

But none of those bills are expected to pass the Senate before the election, and several Republicans have pivoted to a message that highlights not the GOP’s own policies but what they perceive as a potential Democratic overreach: the increasing embrace of more aggressive federal intervention in health care, whether by allowing younger Americans to buy in to Medicare or moving to a single-payer system that would eliminate private insurance.

“It comes down to: Do you want a government-controlled health-care system or do you want to have choices and options?” asked Rep. Tom MacArthur (N.J.), who helped craft the GOP bill that passed the House last year. “That’s what I’m working on. My opponent wants a European-style government takeover. It’s that simple.”

Although few Republicans attack Medicare, the government-run program that covers 44 million seniors, the notion of expanding it has sparked pushback at the highest levels of the GOP.

“What do the Democrats want to do? . . . They want to get rid of private health insurance and have a government takeover of the health-care system. They’ve gone so far left,” House Speaker Paul D. Ryan (R-Wis.) said Wednesday on Fox News Channel.

The midterm campaigns will play out against a shifting health-care landscape, as a result of Republican moves that have largely served to discourage enrollment in the exchanges established under the law. That has prompted insurers to anticipate a disproportionately older and sicker population, leading them to raise premiums ahead of the Nov. 1 beginning of enrollment for next year’s plans.

Nearly a year ago, federal health officials announced that they would slash spending for advertising and other outreach intended to encourage Americans to sign up for ACA health plans by 90 percent, while cutting aid to grass-roots groups helping consumers enroll by 40 percent. In October, the Trump administration ended subsidy payments to insurers, totaling about $7 billion last year, meant to offset discounts they are required to give lower-income customers for deductibles and other out-of-pocket costs.

Congress undid one of the ACA’s pillars in December, when as part of the wider GOP tax bill it zeroed out the tax penalty for individuals who do not purchase insurance — eliminating a key mechanism forcing younger and healthier Americans into the exchanges. The administration also has been working to widen the availability of less expensive health plans that skirt the ACA’s requirements that insurers offer a minimum level of benefits and cover preexisting conditions.

And in June, the Justice Department filed a brief in a federal lawsuit arguing for the first time that the ACA’s requirement that insurers must cover enrollees with preexisting conditions — the most popular part of the law — should be discarded on constitutional grounds.

Late Tuesday, the administration backtracked on a recent move following an outcry from the health insurance industry. The reversal involves about $10 billion in “risk adjustment” payments meant to even out costs between insurers with customers who need expensive medical care and those with healthier customers.

Seema Verma, administrator of the Centers for Medicare and Medicaid Services, froze the payments on July 7, citing pending litigation, but reinstated them Tuesday.

Still, nearly 12 million Americans signed up for ACA plans in 2018, and more than three-quarters of them are receiving federal subsidies to afford them. That means that in any given congressional district, tens of thousands could see their health care under direct threat from the GOP policies — especially middle-class suburban voters whose incomes make them ineligible for subsidies and thus more sensitive to price increases.

Democratic candidates are now trying to capi­tal­ize in a spate of TV ads: Kim Schrier, a pediatrician running in a suburban Seattle district, promises to “stop Trump’s attacks on our health care” and allow more Americans to access Medicare. Cort VanOstran , running in suburban St. Louis, is telling voters that when Republicans “tried to take health care away from millions, I couldn’t sit back.” And Antonio Delgado, running in New York’s Hudson Valley, is highlighting Rep. John Faso’s caught-on-video pledge to an ailing constituent not to take away coverage ahead of last year’s ACA repeal vote.

Faso said in an interview that the bill would have done nothing to undermine coverage for people with preexisting conditions and that he was comfortable running on the “incremental” things” Republicans have done since, such as reauthorizing the Children’s Health Insurance Program and pursuing a reinsurance program for high-cost patients. And he said he was happy to contrast his views with Delgado’s support for a more aggressive federal role in health care.

“People on the left want the government to provide everything,” Faso said. “I don’t believe in that, and I don’t subscribe to that.”

Larry Levitt, a senior vice president at the nonpartisan Kaiser Family Foundation, said the political risks for Republicans are clear. “They have tried to unravel protections that people hold dear while not successfully coming up with a replacement plan of their own,” he said.

Rep. Garland “Andy” Barr (R-Ky.), one of the incumbents targeted this week by the House Majority PAC, said that he, too, is ready to focus on GOP support for community health centers, medical research funding and combating opioid abuse — and that he is confident that voters will not ignore the Democratic attacks.

“Look, I know what their narrative is. But guess what? I’ve tried to fix it. It came up short in the Senate,” he said of the vote last July. “Let me tell you, the failures of the health-care system right now are the failures of Obamacare. That’s my message.”