Bernie Sanders made a series of important admissions during an interview last week with CNBC's John Harwood.

In the interview, Sanders acknowledged that, even if we adopted all his currently proposed options to pay for Medicare-for-all, they would only cover half the amount needed. But he also said he has no intention of putting forward additional revenue plans. "You're asking me to come up with an exact detailed plan of how every American — how much you're going to pay more in taxes, how much I'm going to pay. I don't think I have to do that right now," he said.

Taken together, the statements strongly imply that Sanders, perhaps in contrast with Elizabeth Warren, has a calculated realism about the possibility of actually getting his Medicare-for-all bill passed if he is elected. They also reveal how Sanders' campaign strategy is prioritizing electoral success over advancing Medicare-for-all.

Theoretically, Sanders could do Medicare-for-All with his current funding plan by simply accepting a huge increase to the deficit, a la the 2017 Republican tax cuts but significantly larger. But Sanders also shut down that possibility, claiming, "we will pay for every nickel of Medicare-for-all."

Sanders' promises about his yet to be defined pay-for plan aren't making coming up with an eventual package any easier. Medicare-for-all, he claims, "will save the overwhelming majority of the American people [money], who will no longer pay premiums." Given the massive and often lopsided age, income, health status, and geographic variation in what Americans pay for health care, that is not an easy goal.

The other important element of the interview is how Sanders claims he will get conservative Democratic senators to vote for his bills. He promised to go down to their states, like Sen. Joe Manchin's West Virginia, to rally public pressure against them. "Now they're going to have to think, 'If I don't support an agenda that works for working people, I'm going to have President Sanders coming to my state and rallying working-class people.'"

So, Sanders says his Medicare-for-all bill must be fully paid for to pass Congress. He also admits that, in order to pass any big part of his agenda, he needs each finalized bill to be so popular he can rally the public to pressure senators, even in more conservative states. Yet Sanders, despite having worked for decades on the issue, still can't find a way to finance his Medicare-for-all that meets his promise and that he thinks would be popular enough to run on during a presidential campaign.

If that's the case, that any potential Medicare-for-all funding plan would be a campaign liability, there is no way such a financing plan would be so popular it would put pressure on Democratic senators in places like West Virginia and Montana before a midterm election. If you don't think you can beat Trump with the funding plan in 2020, you can't scare any Democrats into supporting it in 2022. Sanders won't even be able to claim a proper mandate for an actual Medicare-for-all if he doesn't run on a complete package.

Either there is a workable way to fund Medicare-for-all that would keep the whole package broadly popular or there is not, and the fact that Sanders hasn't found one yet is not a good sign. Elizabeth Warren's new plan to finance Medicare-for-all just highlights the problem. It calls for bigger provider cuts than Sanders seems willing to talk about right now, cuts that could increase opposition from doctors and hospitals. Her idea of making employers pay 98 percent of what they currently contribute to private insurance premiums sounds decent politically, but is a mess in terms of workability. It punishes companies who have been offering good insurance, rewards those that haven't, and creates a massive incentive for companies to start dramatically cutting health benefits or spin off workers as "independent contractors" if it looks like the bill might pass.

Sanders likely hasn't fully funded his bill because he can read the polling as well as anyone else and he knows that tax increases significantly drag down overall support. Sanders could be using this presidential campaign to build support for a funding plan, but he is purposely not. Ignoring the hardest part of Medicare-for-all is good for Sanders' overall chance for winning the presidency, but very bad for the long-term prospects of his bill.

Of course, if one's main goal of taking a position is to move the conversation or gain maximum leverage in future negotiations, you don't ever need to work out all the details because the plan will never be fully adopted. And when it comes to moving the conversation, Sanders has succeeded remarkably well. His Medicare-for-all bill has driven much of the media coverage of the 2020 primary, not to mention the debates. Among the major Democratic candidates, Joe Biden has the most conservative health care plan and it is well to the left of Hillary Clinton's 2016 campaign proposal.

Eventually, however, to pass any legislation, you need to sell the public on the entire policy, painful trade offs and all.

Want more essential commentary and analysis like this delivered straight to your inbox? Sign up for The Week's "Today's best articles" newsletter here.