In 2014, Republican Congressman Jeff Denham was all about axing the Affordable Care Act.

He vowed in a campaign ad that aired before that year’s midterm election to “repeal and replace Obamacare so families aren’t forced to pay higher premiums for reduced care.”

Four years later, Denham isn’t talking about the ACA. But his Democratic opponent is — quite a bit.

Josh Harder, who is challenging Denham in the 10th District, which includes the northern San Joaquin Valley cities of Modesto and Tracy, is hammering his opponent for voting to repeal the ACA. The race is neck and neck.

“Jeff Denham told me that (my daughter’s) preexisting condition would be protected under the law,” says a Modesto woman in one of Harder’s campaign commercials. “Then he voted to roll back protections for preexisting conditions and take health care away from millions.”

Democrats in California’s most competitive House races are using their campaign advertisements to pummel their opponents on health care, a topic that nearly three-quarters of voters say figures highly in their choices for Congress this year.

In some commercials, the Democratic candidates share their families’ health care ordeals, but mostly, they use the airtime to remind voters of their GOP opponents’ efforts to repeal and replace Obamacare, including an attempt last year that experts say would have left millions uninsured and watered down protections for people with preexisting conditions.

Republican incumbents, by contrast, are mostly keeping mum about health care in their ads, which tend to focus on the economy, taxes, water and other issues.

Some, however, are following the strategy of President Donald Trump and embattled candidates in other states, insisting in their ads that they do, in fact, support protections for preexisting conditions — despite earlier positions to the contrary.

“There is tremendous irony in this,” said Robert Shrum, a political advertising expert and Democratic strategist based in Los Angeles.

The ACA, Shrum explained, cost Democrats seats in 2010 and 2014; now it’s their power issue. It could resonate loudly in California, which has aggressively defended the law at both the statewide and national levels.

In the past, “people were afraid of what the health care bill meant. They’d heard all the scare stories, so it was a negative,” Shrum said. “Now it’s a tremendous positive for Democrats.”

Indeed, health care is the dominant topic in political advertising this year, according to an analysis by the Wesleyan Media Project, which tracks and analyzes political ads. Between mid-September and mid-October, nearly half of all aired ads for federal races mentioned health care.

Five of California’s GOP-held districts — represented by U.S. Reps. Denham, Mimi Walters, Dana Rohrabacher, Steve Knight and Ed Royce, who is retiring — are considered toss-ups by the Cook Political Report, which means either party has a good chance at winning.

All of them voted for the American Health Care Act, the GOP bill approved by the House in May 2017 before failing by one vote in the Senate. The measure would have phased out the Medicaid expansion, reduced the tax credits that help many people buy insurance from ACA marketplaces and allowed insurers in some cases to charge higher premiums to people with preexisting conditions.

Now Democrats won’t let voters forget it.

In one of the most closely watched races in the country, Harder is trying to unseat Denham, who is seeking a fourth term in a district that went for Hillary Clinton in 2016.

So far, Harder — who has proclaimed health care the No. 1 issue in this election — has spent more than any other candidate in California’s competitive House races on broadcast ads: about $2.2 million from late-May to mid-October, according to the Wesleyan Media Project.

In one of his first commercials, Harder explained that his younger brother was born 10 weeks prematurely and accumulated millions of dollars in hospital bills. Denham’s vote to repeal and replace the ACA could have cut protections for people with preexisting conditions, like his brother, Harder said in the ad.

Ann Crigler, a political science professor at USC, said it’s no surprise that the topic of pre-existing conditions is so prevalent in political ads because it’s an issue almost everyone can relate to.

A Morning Consult/Politico poll in September showed that 81 percent of registered voters nationally believe that insurance companies shouldn’t be allowed to deny coverage for people who have preexisting conditions.

The issue resonates with so many voters — across party lines — that some Republicans also are dedicating campaign ads to it.

In one commercial, Rohrabacher, who represents the Huntington Beach area in Orange County, talks about his daughter’s leukemia diagnosis. Even though Rohrabacher voted to repeal the ACA, which may have resulted in fewer protections for people with preexisting medical conditions, he pledges in the ad to be their champion. “That’s why I’m taking on both parties, and fighting for those with preexisting conditions,” he said.

This declaration shows that Rohrabacher is feeling the heat, Shrum said.

“That’s not an ad on offense, that’s a purely defensive ad,” he said.

Nearby in the 45th Congressional District, located in inland Orange County, Walters is facing UC Irvine law professor Katie Porter, a Democrat who supports “Medicare-for-all,” which would cover all Americans under the public program that is now primarily for people 65 and over.

In one of her ads, Porter moves beyond the ACA and preexisting conditions to highlight Republican attempts to defund Planned Parenthood.

“Walters supports Trump’s scheme to defund Planned Parenthood and restrict women’s access to birth control,” one Porter ad says.

Now that voters are getting bombarded by political health care ads, the question is whether the messages will stir up enough anger or fear to push people to the polls, Crigler said.

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More of Bay Area reaps rewards from state’s ‘Project Homekey’ to buy motels for homeless Stephen Routh, a political science professor at CSU Stanislaus, said research shows that negative emotions are more likely than positive feelings to result in action. And Democrats, he added, are landing some effective blows on health care, stoking fears that people could lose their health coverage.

“Democrats recognize this is a really vulnerable spot in the Republican Party agenda,” he said. Though it’s difficult to break down voter turnout by issue, he predicted that health care “for sure is going make some level of difference.”

This story was produced by Kaiser Health News, which publishes California Healthline, an editorially independent service of the California Health Care Foundation.