During the 2016 presidential campaign, Donald Trump proclaimed what has become almost a clarion call to his supporters and detractors alike, "I could stand in the middle of Fifth Avenue and shoot somebody, and I wouldn't lose any voters." Although he hasn't followed through on that experiment, the President has taken many positions that would seemingly prompt him to lose some of his support.

Polls show his approval rating ranging consistently between 35 and 45 percent. And among the GOP his support is especially strong. The most recent polls show that 84 percent of Republicans approve of his job - almost the same level of support he had among Republicans during the first weeks of his presidency.

Despite resignations of key administration officials, an ongoing investigation about his campaign and controversies about his alleged relationships, he remains largely unscathed.

But one key issue - health care - threatens to erode his support.

"We're going to have insurance for everybody," Trump told the Washington Post after he was elected. "There was a philosophy in some circles that if you can't pay for it, you don't get it. That's not going to happen with us."

So far, that pledge hasn't become a reality.

Earlier this year, my organization, the Texas Medical Center, polled 5,000 people across the country on a range of hot-button health policy issues. We asked the 37 percent of respondents who said they're planning to vote for Trump in 2020, "How likely is it that President Trump's actions on health-care policies, specifically his support for increasing the number of uninsured, will keep you from voting for him?"

Overall, we found a 39 percent reduction in the number of Trump supporters who'd support him for re-election if the number of uninsured increased. That could easily prove to be the difference between winning and losing.

In addition, the survey asked about provisions of the GOP's American Health Care Act, which would have abolished any previous changes to Medicaid expansion among other changes. In fact, a nonpartisan Congressional Budget Office analysis of the White House-endorsed plan showed it would cause 24 million people to lose insurance - a far cry from "insurance for everybody."

The bill fell a single vote shy of passing.

About 90 percent of respondents in our survey said that Medicaid should be extended to all poor adults, and 82 percent don't want to limit the amount Medicaid pays for a sick patient. It is possible that if that bill had passed, Trump would have lost a large number of supporters.

So what's the takeaway? Despite his ability to persevere through almost any controversy, Trump shouldn't consider his supporters' loyalty a given. At a time when it may seem that politics rather than policy dominates the discourse, a wrong move on a key policy issue - health care - could be the president's undoing. We know that the vast majority of Americans are in favor of decreasing the number of uninsured, and they'll keep that issue in mind when they head to the polls.

Health care, of course, was a pivotal issue in the 2016 campaign. It is in 2018 too, and it almost certainly will be in 2020. As Americans increasingly struggle with health-care expenses, it seems that what was once thought to be a winning strategy - increasing the number of uninsured Americans - should finally be scrapped by lawmakers, including Trump, if they hope to win reelection.

The president might be able to get away with shooting someone on Fifth Avenue without losing votes. But he might not be able to survive a misstep on health-care.

Dr. Arthur Garson is a former medical school dean, past president of the American College of Cardiology, and currently director of the Texas Medical Center Health Policy Institute in Houston. Ryan Holeywell is the director of communications for the Texas Medical Center.