In fact, Trump himself seems to be experiencing cognitive dissonance. “The rich will not be gaining at all with this plan,” he declared Wednesday. Like his claims that Trumpcare wouldn’t cause anyone to lose coverage, this statement raises questions about what’s going on in his mind: Is he oblivious, lying, or both?

But in any case, such statements are going to make it even harder to pass anything: The contrast between what he’s claiming and anything Republicans in Congress will be willing to support is so great as to practically invite ridicule and another popular backlash.

I’d add that tax cuts for corporations and the rich have little popular support. Even many self-identified Republicans, especially among the working-class voters who supported Trump, tell pollsters that corporations and the wealthy pay too little, not too much. Trump seems to imagine that he can rally broad voter support for his tax plans, but it’s hard to see how.

But is the push for single-payer health care taking Democrats down a similar path?

Unlike just about everything Trump and company are proposing, Medicare for all is a substantively good idea. Yet actually making it happen would probably mean facing down a serious political backlash. For one thing, it would require a substantial increase in taxes. For another, it would mean telling scores of millions of Americans who get health coverage though their employers, and are generally satisfied with their coverage, that they need to give it up and accept something different. You can say that the new system would be better — but will they believe it?

Such concerns may not seem very salient right now: Given Republican control of the White House, single-payer is going to be at best an aspiration for the next three-plus years. But what if rigid support for single-payer — as opposed to somewhat flexible support for universal coverage, however achieved — becomes a litmus test? In that case, Democrats could eventually find themselves facing a Trumpcare-type debacle, unable either to implement their unrealistic vision or to let it go.

The point is that while unrealistic promises may not hurt you in elections, they can become a big problem when you try to govern. Having a vision for the future is good, but being real about the difficulties is also good. Democrats, take heed.