“I’ll be the first to admit I’m as surprised as anyone to be in this position,” said Brad Woodhouse, the executive director of the health care advocacy group Protect Our Care . Mr. Woodhouse worked at the Democratic National Committee when the Affordable Care Act passed in 2010, and saw Democrats struggle to discuss it in the years afterward. “But every poll we take, it’s the issue that’s most important to people.”

The newfound Democratic health care enthusiasm may not translate into victories, of course. Republican campaigns are turning away from health care as a major issue this cycle, planning to pay more attention to other messages like immigration, job creation and attacks on Democrats. When they talk about health care, many candidates are focusing on Democratic efforts to expand public health insurance coverage, not on continued calls to repeal Obamacare.

“People tend to vote with their pocketbooks, and the strong economic performance will be at the forefront of every discussion this fall,” said Jesse Hunt , a spokesman for the National Republican Congressional Committee , adding, “A contrast between single-payer health care and our ideas — a more patient-centered approach — is a debate we fully welcome.”

But some Republicans are sticking to the old anti-Obamacare script. In Missouri, Ms. McCaskill’s expected opponent, Josh Hawley, the state attorney general, talks about health care frequently. “I talk about Obamacare and just the broken health care system,” he said, noting that the rising cost of health insurance is one of several factors eroding middle-class security.

Two main changes appear to have turned the tide on health care as an issue. The Republican effort to repeal Obamacare last year made the embattled health law more popular than it has ever been. For the first time, the Affordable Care Act earned the support of a majority of Americans in public polls, a small shift but one that has been durable, even as Congress moved on to a tax overhaul and other issues. Pre-existing condition protections have always been much more popular than the law over all.

The threat of repeal appears to have been particularly galvanizing for Democratic activists, who came out to protest and contact their legislators during the debate.

The second change came more recently, when the Trump administration decided not to defend Obamacare from a lawsuit brought by the Republican attorneys general of 20 states. The lawsuit argues that the entire law should be invalidated as unconstitutional. The Trump administration’s position is that most of the law should remain on the books, but that its protections for people with pre-existing illnesses should be stripped away.