Chicago (CBS) — There’s one issue where Democrats nationwide have chosen to focus their attack on Republicans. It’s health care.

And Republicans who voted to kill Obamacare are now on the defensive about the collateral damage.

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The issue is present in Chicago’s two key congressional races.

Democrat Lauren Underwood is hitting Republican Congressman Randy Hultgren hard over health care. And so is challenger Sean Casten in his bid to unseat GOP Congressman Peter Roskam.

The Democrats’ consistent health care theme is no accident. It’s a strategy.

“Health care keeps coming up atop or even at the top of the issues voters are telling us they care about,” said Anthony Salvanto, CBS News director of polling.

House Republicans’ repeal of the Affordable Care Act, or Obamacare, also wiped out guarantees of coverage for patients with pre-existing conditions.

“We ask people what they think of the changes the recent Republican Congress has made to the health care system, and by and large people say they don’t feel like they were helped by it,” Salvanto said.

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So, Democrats are piling on with full force.

“Peter voted 62 times to repeal the ACA,” Casten said during a public television forum with Roskam. “If he’s for protecting pre-existing conditions, that’s a newfound position.”

In response, Republicans like Hultgren and Roskam say GOP health care proposals have the same guarantees.

“We’re going to protect people with pre-existing conditions,” Roskam said at the same forum. “We’re going to make sure there’s no lifetime caps, but we’re also going to embrace more flexibility.”

The problem is, they’re just that – proposals. No health care plan was ever passed by the Republican-controlled Congress, which leaves the GOP vulnerable.

“I think that, probably from a strategic standpoint, is probably making the Democrats want to run on that issue,” Salvanto said.

The Obamacare repeal didn’t become law due to the late Sen. John McCain’s vote in the Senate. Still, that House repeal vote provided Democrats with a potent issue.

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Republicans have proposed a bill to protect patients with pre-existing conditions. But, some experts have said those safeguards aren’t as strong as under Obamacare.