Sen. John McCain (R-Ariz.) speaks to supporters in Phoenix on Aug. 30. McCain’s campaign has seized on problems with his home state’s health insurance exchange as he fights Rep. Ann Kirkpatrick (D) for a sixth term. (AP Photo/Ross D. Franklin, File)

Republicans have found an issue on which they can play a rare bit of offense in their quest to hang on to their Senate and House majorities: Obamacare.

Criticism of the landmark health-care law has been a staple of GOP campaigns since its party-line passage in 2010. But unlike six years ago, in the first election after the passage of the Affordable Care Act, Republicans aren’t running a national campaign against federal government overreach. Instead, they’re lobbing localized attacks in key states on issues plaguing the state insurance exchanges mandated by the law.

Republicans and their well-funded allies are seizing on departure of major insurers from places where they are losing money and on health insurance rate hikes that could hit voters just days before Election Day in states with competitive Senate races.

For Republicans, it’s an issue that resonates deeply with their base and allow them to buffer any downdraft from GOP  presidential nominee  Donald Trump.

Democrats need to win four Senate seats to retake the majority if Hillary Clinton wins the election. And the GOP is playing defense in several states such as Pennsylvania, New Hampshire and Illinois, which President Obama won in 2012. Polls show a tightening presidential race, with Trump now leading in Florida and Ohio, which could help Sens. Marco Rubio (Fla.) and Rob Portman (Ohio), who are ahead in their races there.

Two national insurers, UnitedHealth and Aetna, have drastically scaled back their participation in the exchanges across the country, citing financial losses. And in dozens of states — including Pennsylvania, New Hampshire and Indiana — insurers have sought major rate hikes that could hit voters just before Election Day.

“This is financial disaster, and it’s anything but affordable, so it’s time that we have a debate about what we move to,” said Sen. Richard Burr (R-N.C.). He is one of four senators locked in difficult reelection battles who have co-sponsored a Senate bill that would suspend tax penalties for individuals who live in states where average insurance premiums have increased by 10 percent or more, essentially exempting them from Obamacare.

One analysis of rate proposals, by the McKinsey Center for U.S. Health System Reform, found an average national premium increase of 11 percent for plans that will go on sale Nov. 1.

The issue is starting to show up on the airwaves in a big way. It has been a centerpiece in the U.S. Chamber of Commerce’s advertising for Senate races in Arizona, Indiana, Pennsylvania and several other states.

Rob Engstrom, the Chamber’s national political director, said that the recent problems have helped transform what had been a broad national issue into one with special resonance in states that have seen major rate hikes and insurer pullouts.

“We believe local beats national every day of the week,” Engstrom said. “The impacts of Obamacare, in many places across the country, they are deeply personal, and they are kitchen-table issues. People feel betrayed. They were promised they could keep their doctor. They were promised they could keep their coverage. … The opposite has occurred.”

Nowhere has health care been so dominant an issue as in Arizona, where Sen. John McCain is seeking a sixth term and insurer pullouts have been especially drastic. Seven insurers have stopped selling policies in the state’s largest county, leaving at this point only one company selling insurance to those not covered by employer insurance or government plans like Medicare or Medicaid.

McCain’s campaign has hammered away at his Democratic opponent, Rep. Ann Kirkpatrick, for her support of the law.

A recent McCain ad ticks off the numbers of consumers now lacking options in the state’s biggest counties. “Providers are leaving,” says the ad’s narrator as Kirkpatrick is pictured walking away from a camera. “Her proudest vote puts us all at risk.”

Big spending on this issue is expected not only from the campaigns and from the Republican Party but from the Koch donor network that has pledged to invest heavily in maintaining a GOP Senate majority.

In one hint of the messaging to come, Americans for Prosperity — the Koch group focused on grass-roots organizing — recently targeted Wisconsin voters with a mailer tying Russ Feingold, the former Democratic senator seeking to regain his seat, to Obamacare’s passage. Sen. Ron Johnson, Feingold’s Republican opponent, has joined Burr, McCain and Sen. Kelly Ayotte (R-N.H.) in co-sponsoring the tax-penalty exemption bill.

“A lot of the rate hikes are just happening now,” said James Davis, a spokesman for Freedom Partners, another Koch group that pours money into TV ads.

In another Senate battleground, Indiana, state regulators have approved rate hikes as high as 41 percent for individual policies on sale for next year. A super PAC with ties to GOP leaders launched an ad last week tying Evan Bayh, another former Democratic senator seeking to regain his seat, to his Obamacare votes in 2009 and 2010, while Americans for Prosperity has focused on a similar message in its field work in the state.

The health-care issue has yet to show up in a big way in House races, but top Republicans say it is just a matter of time before their candidates start highlighting it.

“It will be an issue that you will see rising in October,” said Rep. Steve Stivers (Ohio), a deputy chairman of the National Republican Congressional Committee. “If you have a major insurer pull out of a plan that affects your district, and 40 or 50 percent of the market share is in that plan, it’s going to be an issue.”

In recent interviews, Republican candidates in close races were quick to bring up Obamacare; Democrats tended to talk about it only if prompted.

“If the race is about the ACA, we win,” said Jason Lewis, a former radio host running as a Republican for a seat in the Minneapolis suburbs.

Like the Republican nominee for president, down-ballot candidates are convinced hey can criticize the law’s failings without getting entangled in a debate over how to fix them.

One of the ACA measures meant to ease the transition was a “risk corridor,” to reimburse insurance companies that wound up with too many costly, high-risk customers. When he was considering a bid for president, Rubio passed an amendment limiting the government’s ability to “bail out” insurers in the corridor.

Far from generating a backlash, it became a pro-Rubio talking point, and the issue has not surfaced again in his Senate reelection bid. But conservative activists on the ground see the Obamacare story coming up again and again as they knock on doors.

“It’s one of the biggest issues in Florida,” said Tim Phillips, the president of Americans for Prosperity. “Those premium increases are outraging a lot of folks, frankly even folks who are not in Obamacare. They fear the implosion of Obamacare will harm their own plans.”

Americans for Prosperity has focused most of its attention on Florida, Indiana, Pennsylvania, Wisconsin, Ohio, and Nevada. Obamacare repeal messaging is part of its pitch, and there’s no sign of a backfire.

“The baseline poll number we’ve been watching is on the question: ‘Has your family had any interaction with Obamacare?’” said Phillips. “As that number goes up, the negative number goes up, too. That’s told us this issue is going to be salient for a very long time.”

Democrats, meanwhile, betray little concern that the sputtering exchanges will be a win-or-lose issue come Nov. 8. About 13 million Americans are enrolled through the exchanges — more than 10 times that number have employer-provided insurance, and another 58 million are enrolled in Medicaid.

And they are quick to point out that Republicans have long been fond of criticizing the Affordable Care Act without putting forth a fiscally vetted alternative.

“The bottom line is that the Republican stance — repeal and have no solutions — isn’t going to serve them well,” said Sen. Charles E. Schumer (N.Y.), who leads the Senate Democrats’ messaging efforts.

Ten Democratic senators, including Schumer and two other top caucus leaders, last week backed a resolution calling for a “public option” on the exchanges. That proposal for a government-managed plan that would compete with the private insurers was dropped by Democrats in 2009 in order to get the larger bill passed.

Sen. Sherrod Brown (D-Ohio), a longtime proponent of the public option, said there are also popular aspects of Obamacare — including prescription drug coverage for seniors that “are very powerful answers to their little it’s-spinning-out-of-control thing.”

Asked what he would advise Democrats to say if Republicans called the law hopelessly broken, Brown suggested that they dare Republicans to fix it.

“Let ’em pass the public option,” he said.