Across the country, Democratic ads are telling voters a big lie. Dems claim they’re protecting people with pre­existing medical conditions but Republicans would take that protection away. The idea is that ObamaCare is the only way to safeguard people with preexisting conditions. That’s false, and the outcome of the midterm elections could turn on this falsehood.

A super PAC allied with Sen. Chuck Schumer is behind many of these ads, including one targeting Republican Josh Hawley, who’s in a tight race with Missouri Sen. Claire McCaskill. If he’s elected, the ad claims, millions of Missourians could lose their protections. In another ad, McCaskill tells the camera “Two years ago, I beat breast cancer,” then says her opponent would do away with protections for preexisting conditions like cancer.

These ads blatantly mislead voters. Hawley’s on the record insisting that health-reform legislation must include these protections.

So is Wisconsin Gov. Scott Walker, who has called on his state lawmakers to enact protections. But never mind the truth. The Democratic Governors Association is funding an ad with a breast-cancer patient saying she won’t be able to afford lifesaving treatments if Walker is reelected.

A whopping 81 percent of voters, including 80 percent of Republicans, support these protections, polls show. That’s why the GOP’s ObamaCare-repeal bill safeguarded these people by providing funding for each state to cover insurance for their sickest patients. But Republican pols failed miserably last year to explain that, opening the door for Democrats to vilify them as enemies of people with health problems.

President Trump could stop these shenanigans and also calm Americans’ anxieties with a new, free-standing reform. What’s needed is a federal fallback-insurance plan, aggressively promoted by the president, that will cover anyone turned down by an insurer. Period.

ObamaCare isn’t the only way to protect people with preexisting conditions. Just the most unfair way. It compels insurers to charge the healthy and the sick the same price. That’s the major reason ObamaCare premiums for 2019 are triple what they were in 2013.

A federal fallback-insurance fund will guarantee coverage for chronically ill people, while letting premiums return to sane levels for healthy buyers. It will pay the medical bills of those with preexisting conditions out of general revenues — not abandoning them, but lifting the burden off premium-payers in the tiny individual market and spreading it more broadly.

We already know how to do it. In 2010, a temporary federal Pre-Existing Condition Insurance Plan was established for people turned down by insurers as a stopgap until the ObamaCare exchanges opened.

What will a permanent fund cost? Based on the $32,000 spent per patient in the temporary fund, plus an adjustment for inflation, and predicting 400,000 to 500,000 enrollees at any one time, it’s likely to cost taxpayers roughly $20 billion a year. That’s money well spent helping the sick. And it’s less than half the current cost of subsidizing healthy people to buy overpriced ObamaCare plans.

Right now, the middle class, who are ineligible for a subsidy, are getting priced out of ObamaCare. They’re hoping to enroll in so-called short-term plans that offer fewer benefits (no inpatient mental-health care, for example) and low prices. The Trump administration recently relaxed insurance regulations to help sticker-shocked consumers buy these plans.

Yet last Friday seven advocacy groups, such as the American Psychiatric Association, sued to stop these plans and slam closed this escape hatch from ObamaCare. The litigants said the plans would “draw low-risk people out of” ObamaCare. That’s exactly the point. People want choices and lower premiums.

But Americans also want protections for people with preexisting conditions, a goal the president is on the record supporting. A federal insurance-fallback program will accomplish that. Now is the time to get it done, and stop the demagoguery about preexisting conditions.

Let voters choose based on real issues, not a phony one.

Betsy McCaughey is a senior fellow at the London Center for Policy Research.