Something was different in September 2017 when Sen. Bernie Sanders held a press conference to unveil his latest version of single-payer, or “Medicare for All,” legislation. The last time he had introduced such legislation, a couple of Congresses earlier, he had done so alone: His bill in 2013 had zero co-sponsors.

In 2017, though, 16 senators joined him. Among those new co-sponsors were four who had come around to the idea of public health care financing just as they were considering bids for the presidency: Sens. Cory Booker, Kamala Harris, Elizabeth Warren, and Kirsten Gillibrand. The Democrats had just beaten back a Republican effort to repeal Obamacare, and it seemed like the time to look for the next step toward universal coverage. Candidates looking to 2020, meanwhile, recognized the energy that Sanders’ bold 2016 campaign inspired and hoped to capture some of that for themselves. Support for a single-payer health care system was poised to become the default position of the Democratic presidential field.

It hasn’t worked out that way. As of now, among the contenders who have even a microscopic chance of winning, only Sanders and Warren still support single-payer health care legislation, the former more effusively than the latter. The others have settled on supporting some version of a public health care option or a Medicare buy-in. With Sanders and Warren locking down so much of the left vote that prioritizes single-payer, the calculus for candidates in the middle is to wield the arguments against single-payer—its cost, the middle-class tax hikes it would necessitate, and the elimination of existing insurance arrangements—rather than be subject to them.

As the field has shifted from support for “Medicare for All” to opposition, it’s made life difficult for candidates whose commitment on either side of the question has been murky. Of the five leading candidates—Warren, Joe Biden, Sanders, Buttigieg, and Harris—only Sanders and Biden have demonstrated absolute comfort in the positions they’ve staked out. Sanders has always been for it; Biden has always been against it. For the others it’s either been, or is becoming, a mess. Dabbling in support for single-payer, without being prepared to defend it honestly and authentically, has a nasty way of leaving a mark on one’s credibility.

When deciding whether to support Sanders’ single-payer legislation, one of the first things the future candidates should have considered is whether they supported and were willing to defend Section 107. This is the part of the legislation that makes it “unlawful” for an insurer to sell, or an employer to provide, “duplicate” benefits to those granted in the new government program. Since the benefits in Sanders’ bill are comprehensive—exempting, as Sanders has described it, only things like elective cosmetic surgery—this is the section of the bill described in shorthand as “banning private insurance.”

And yet Harris, one of the co-sponsors of the bill, repeatedly struggled to answer questions about this provision in the early stages of her campaign. Days after she entered the race in January, during a CNN town hall, Harris answered affirmatively that she supported eliminating private health insurance, saying “Let’s eliminate all of that. Let’s move on.” A day later, Harris’ team clarified that she was open to other ideas as well.

This wasn’t the last time she would do this. In the first Democratic debate in June, Harris joined Sanders in raising her hand when asked whether she would ban private insurance in favor of a government plan. The next day, she argued that she had misheard the question and was being asked whether she would get rid of her own private health insurance plan in favor of a government one. The question had been awkwardly phrased, but it was also a repeat question from the presidential debate the night before, when only the single-payer supporters had raised their hands. In order to move on from the question Harris released her own plan in July , which created a 10-year path to “Medicare for All” that still allowed space for private insurers to compete, claiming that she had changed her mind about Sanders’ bill only following conversations she had across the country. Sure.

Warren is to Harris’ left and explains her support for single-payer in a way that’s convincing enough to demonstrates she believes it’s a good idea. But believing single-payer is a good idea is far from treating it as a priority, or something you might spend enormous sums of political capital fighting for as president. Warren gets jazzed about using her signature item—a wealth tax—to cover the cost of plans for universal child care and education, and on using executive power to clean up corruption and regulate corporate America. On health care, she’s simply “with Bernie” and would prefer to be asked no follow-up questions.

Unfortunately for her, people—specifically cable news debate moderators and her fellow candidates—do have follow-up questions, and the first 45 minutes of every presidential debate are spent on them. The question that trips up Warren is not the future of private health insurance, but whether middle-class taxes will have to go up to cover the national program. Since Sanders’ plan eliminates cost-sharing in the form of deductibles, premiums, and copays, the answer is yes, even if overall health costs for the average American will go down. Unlike Sanders, though, Warren refuses to concede that middle-class taxes will have to go up when asked about it point-blank, presumably scared of the clip coming back to hurt her in a general election. That refusal only ensures that it will continue to be asked. The longer she doesn’t have an answer, the more evasive she looks.

It’s a pickle for Warren, and last weekend she announced that her path out would be to release a financing plan for “Medicare for All.” She could release a plan that’s bullshit and doesn’t come even close to financing the system, but does avoid tax hikes. She could release a plan that does include tax hikes, ensuring that a policy that’s not even a top priority for her will still provide a talking point against her in the general election. She could release a plan that retains some modest premiums, deductibles, and copays, but gives Sanders a wedge to use against her on the left. Finally, she could release a time machine that allows her to go back to 2017 and try this again, without winging it.

Warren’s most notable antagonist on single-payer, meanwhile, has been Buttigieg, who’s hit Warren both on the debate stage and in advertisements, of which he has the resources to fund many. But Buttigieg’s own blurry past on single-payer makes his criticisms of it now come across as conniving in their own right. In a Twitter conversation in early 2018, Buttigieg claimed that he’s supported “Medicare for All” since 2004, linking to a Harvard Crimson column in which he voiced his support for a “single-payer health care system.”

When pressed further to firm up his support, he tweeted, “I, Pete Buttigieg, politician, do henceforth and forthwith declare, most affirmatively and indubitably, unto the ages, that I do favor Medicare for All, as I do favor any measure that would help get all Americans covered.” And in a February 2019 appearance on Morning Joe, Buttigieg defended it as a “compromise” between the nationalized health care of the U.K. and the right-wing’s free-market vision of health care.

Buttigieg has argued that he’s been consistent about his preferred health care approach—“Medicare for All who want it”—for the duration. But you don’t flip from defending single-payer health care to using the arguments against it as your go-to cudgel for advancement without taking a gander at the polls. Warren might have a response about Buttigieg’s own opportunism the next time he tries to lean into her.

If the presidential debates were limited to Sanders and Biden (or Amy Klobuchar or Michael Bennet, if they were competitive), we might only see moderators spend a cool five or 10 minutes on health care, maybe in the middle or back end of a debate. The candidates would lay out their positions and the well-worn talking points associated with them, and then moderators would move on because there would be nothing squirrely to investigate, no illicit past positions to dredge up. Instead, the temptation for candidates to get cute with single-payer health care early in the process has ensured that it’s all they ever spend time discussing again, even when they would rather not.