(CNN) On the campaign trail. In television ads. On Twitter. Everyone -- even the President -- is suddenly talking about pre-existing conditions.

Protecting people who are or have been sick has turned out to be among the most popular parts of the Affordable Care Act, also known as Obamacare -- and it's emerged as one of the top issues in the upcoming midterm election, as Democrats attack Republicans over their continued push to repeal or at least weaken the landmark 2010 law.

President Donald Trump on Wednesday explicitly pledged to "totally protect" people with pre-existing conditions.

Trump's enthusiasm is easy to explain. Three-quarters of Americans say that it is "very important" for the law to continue prohibiting health insurers from denying coverage because of medical histories, according to the Kaiser Family Foundation's September tracking poll -- 58% of Republicans feel the same way. And about the same share of Americans say it's "very important" that insurers continue to be barred from charging sick people more.

Democrats, at the moment, are seen by more voters as the defenders of these protections. Some 58% of respondents said they trust the party more to continue the law's provisions, compared to 26% who chose Republicans, according to a Kaiser election tracking poll released in mid-October. Among independents, some 60% said Democrats would do a better job maintaining the protections, while 19% picked Republicans.

Obamacare revolutionized health insurance for people with less-than-stellar medical histories. Under the health reform law, insurers can no longer deny coverage to consumers or charge them more because of their health backgrounds. Also, carriers are required to offer comprehensive benefits, including maternity care, mental health services and prescription drugs.

"It was a pretty fundamental change," said Erin Trish, associate director of health policy at the University of Southern California's Schaeffer Center. "It made health insurance more available to people with pre-existing conditions at a much more affordable rate."

The intense focus on these provisions in the midterms, however, has left Republicans in the tricky position of defending one of the central tenets of Obamacare, even as they continue to call for its repeal.

At the same time, the Trump administration is taking steps that could hurt those who depend on Obamacare's protections. The Justice Department is arguing that these provisions should be invalidated as part of a lawsuit brought by Republican attorneys general and governors from 20 states. And the administration is making it easier to buy short-term health plans , which have lower premiums but can deny coverage or charge higher rates based on people's health backgrounds.

Promising to protect those with pre-existing conditions also conflicts with another Republican health care promise— lowering costs. The provisions are one of the main reasons why policies on the individual market have become so pricey. Insurers had to raise rates for everyone because they could no longer cherry-pick only healthy applicants and because they had to cover a wide array of services.

Many of the GOP proposals to repeal Obamacare last year would have chipped away at or eliminated the law's ironclad defense of those with pre-existing conditions in hopes of reducing premiums. But people's fondness for this security blanket ultimately helped doom the effort in Congress.

Relatively few Americans actually take advantage of the protections each year because the vast majority are covered by employer or government insurance plans, which generally offer comprehensive benefits and don't discriminate based on medical histories. There are roughly 14.4 million people in the individual market, both on the Obamacare exchanges and outside of them.

However, many people may find themselves turning to individual insurance plans at some point in their lives, especially if they are in between jobs, are employed part-time or as contractors or work at small firms that don't have to provide benefits. Roughly 27% of adult Americans under age 65 have health conditions that would have likely left them unable to get individual market policies prior to the health reform law, according to Kaiser.

"Clearly, everyone has anxiety ... if you have employer-sponsored coverage -- which more than half the country does -- that you may at some point lose that coverage and have to buy a policy" on the individual market, said Doug Badger, senior fellow at the Galen Institute, a right-leaning think tank. "The question is whether an insurer could exclude coverage of your pre-existing medical condition."