Sherrod Brown has been calling for universal health care since 1992. That's when he first ran for a U.S. House seat in Ohio, vowing to decline the federally subsidized insurance for members of Congress until his constituents could get similar coverage. He won that race and he kept that pledge, buying policies on his own until 2011, after the Affordable Care Act became law. He was a senator by that point, and like every other Democrat in the chamber, he voted for President Barack Obama’s signature health care law. But before Brown did that, he promoted a series of proposals designed to make the program more generous and comprehensive. One of them was a last-minute amendment that would have replaced Obamacare’s intricate scheme for competing private insurers with a “Medicare for all” program, under which everybody would enroll in a government-run insurance plan. Nobody seriously thought Democrats were about to scrap legislation they had spent nearly a year writing. By supporting the amendment, Brown was mostly trying to demonstrate his commitment to improving the Affordable Care Act, if not before it became law, then afterward. It was a symbolic act, but a conspicuous one, with only one other senator co-sponsoring it. The amendment’s author was Sen. Bernie Sanders (I-Vt.), the most visible champion of “Medicare for all.” In 2016 he made the idea a centerpiece of his presidential campaign, and a year later, he introduced a new version of “Medicare for all” legislation ― this time, with 16 co-sponsors, proving just how popular the idea had become in the interim.

NBC via Getty Images Sen. Sherrod Brown (D-Ohio) on "Late Night With Seth Meyers" on Jan. 23. Brown, who previously supported “Medicare for all,” now says he wants to focus on narrower ideas, like opening up Medicare to near-retirees, because they are more politically realistic.

But this time Brown declined to join them, explaining in a prepared statement that while he remained “supportive of ‘Medicare for all,’” he preferred to focus on more incremental, potentially bipartisan measures, like allowing people to buy into Medicare as early as their 50s. Now Brown is thinking about a run for president himself. And like every other Democrat who has launched a 2020 campaign or is seriously contemplating one, he has been getting questions about whether he will endorse “Medicare for all.” His answer has been “no,” and he has become more explicit about why, saying he doesn’t think it’s realistic politically. “It’s easy to say ‘Medicare for all’ and make a good speech but see no action,” Brown told Politico in late January, “I want to see action.” This weekend, he told voters at an Iowa forum that “If we can get Medicare to 50, if we can pass Medicare at 55 tomorrow, two things would happen. A whole lot of people’s lives would improve, and a whole lot of voters would think, ‘Oh, that works. The next step is to do more.’” Brown’s posture has already provoked criticism from progressives. If he proceeds with a presidential campaign, he can expect a lot more of it ― as can any other Democratic candidates whose commitment to “Medicare for all” seems suspect. Just ask Sen. Kamala Harris (D-Calif.), who is running for president. After reaffirming on CNN last week that she supports “Medicare for all,” aides made clear (as they did previously) that she remained open to incremental steps or alternative methods of achieving universal coverage. Social media erupted with accusations from activists that she was watering down her position. So far, the only person who seems immune is Sanders, who may join the 2020 race. None of this should be surprising. Presidential campaigns are all about making distinctions among candidates, and there are real differences between somebody like Brown, who doesn’t want to push “Medicare for all” right now, and somebody like Sanders, who does. But the question for Democratic voters over the next year and a half, as they choose their 2020 presidential candidate, is exactly what those differences reveal and, ultimately, which ones matter. The Debate Over ‘Medicare For All’ Is Real Now One thing that shouldn’t be in doubt is that nearly all the major Democratic 2020 candidates believe in universal health care coverage, which has been a core party value since President Harry Truman tried to create a national health program in the late 1940s. Truman had in mind a government-run system that would cover everybody. When his effort crumbled in the face of lobbying by the American Medical Association and other health care industry groups, Democrats began a decades-long effort to get to universal coverage by some other method ― whether it was providing government-run insurance to just part of the population, as Medicare does, or preserving a large role for private insurance, as the ACA does. The tilt toward private insurance coincided with a time when the Democratic Party as a whole had a more conservative orientation and its policy advisers had more faith in private enterprise and the forces of competition. Now interest in government-run insurance appears to be surging, partly because so many people still face high medical bills even with the ACA in place and partly because managing Obamacare’s complex markets has proved difficult, with Republicans constantly trying to undermine them.

Bloomberg via Getty Images Sen. Kamala Harris (D-Calif.) has said she favors “Medicare for all.” But she has also said she's open to other forms of universal coverage and would support incremental reforms on the way to universal health care coverage.