This post is part of Polyarchy , an independent blog produced by the political reform program at New America , a Washington think tank devoted to developing new ideas and new voices.

The next fight over the government shutdown, due February 8, seems likely to center on the twin issues of funding for President Trump’s border wall and a legislative extension of the Deferred Action for Childhood Arrivals immigration program. How much of Trump’s request for $25 billion to fund the wall will Democrats be willing to give up in exchange for DACA? (That’s assuming the egregious and controversial provisions to scale back legal immigration in Trump’s most recent proposal are quickly rejected.)

While the wall would be the most wasteful and damaging use of taxpayer money at this scale since the Iraq War, Democrats will probably be willing to concede significant funding, as they were in earlier negotiations. It’s only money, after all, not the basic human dignity that’s at stake in DACA and other immigration issues. There’s little visible harm to people living in the United States, and the whole process will take years and years, especially to resolve the issues involving private property and eminent domain along the border. Ideally, Democrats will be able to reallocate money from the unspent wall fund when they take control of Congress and/or the White House.

Every so often during this debate, someone will point out that Trump promised Mexico would pay for the wall, and whatever happened to that? Everyone chuckles and goes back to the normal conversation. (Health analyst Topher Spiro has been among the few urging the press and Democrats to treat Trump’s promise seriously.)

We laugh because none of us with any experience in government or policy took Trump’s promise seriously for a minute. We can’t force another country to pay for a project in the US that they don’t want and wouldn’t benefit from. In polls, strong majorities agree, saying they don’t expect Mexico to pay for the wall. But some voters certainly did, including a majority of Republicans in a Quinnipiac poll last May. Surely there are at least a few million voters who supported Trump because they thought a presidential candidate and successful businessperson wouldn’t or couldn’t make a promise like that unless it was possible.

Before even opening talks on an immigration compromise, and after taking the extremist elements of this week’s offer off the table, Democrats should insist that Trump produce a detailed, specific plan, with a timetable, under which Mexico would pay for the project, complete with agreement from Mexican officials.

Of course, that won’t happen, and yes, it seems silly to ask for it, just a kind of rhetorical gimmick. But not to demand that a president take his own promises seriously — just because we don’t take them seriously — is equally problematic.

There’s some inevitable dishonesty in politics — even the best politicians promise what they aspire to achieve, going a few steps beyond what they know they can get done. Was Obama’s promise to govern as a cross-partisan unifier a lie, or just an unfulfilled ambition? What about, “If you like your doctor, nothing will change”?

But there’s a certain level of dishonesty or a type of lie that goes beyond overpromising or shading the truth, and that’s more than just a moral failing by an individual. Lying in politics matters particularly when voters believe the lies and act accordingly. Led to believe things are possible that aren’t, they are not just lied to but defrauded, in the same way they would be if a financial adviser promised that an investment would return 10 percent a year without risk. Political lying of this type obstructs democracy and profoundly deepens the climate of pervasive distrust in which Trump has thrived.

There have to be consequences for lying in politics, and the first place those consequences are found is, of course, at the ballot box. Impeachment, censure, or other quasi-judicial remedies might apply in other circumstances, though not this one. But the best way to make sure that big lies, completely unfounded in aspirations or real possibilities — such as “Mexico will pay for the wall” — have consequences is to treat them with utter seriousness and demand that elected officials live up to them or admit they were lies.

It’s tempting and easy to laugh off such an absurd claim as “Mexico will pay for the wall,” but for the health of democracy, we have to resist that temptation.