“Nothing matters.” This bleak sense that Donald Trump could not be stopped became a refrain for battered progressives when he was first elected four years ago. Now, with the next election a month away and long-buried tax returns unearthed by the New York Times, the phrase is undergoing a worrisome revival. The returns reveal the president as a failure and a possible fraud, yet few people seem to believe in the power of this revelation to provoke any meaningful reaction. The bombshell, many argue, will make nary a dent in Trump’s popularity.

Maybe. People who trust a newspaper like the New York Times probably don’t trust Trump, as a rule. People who hadn’t counted him out while he was hiding his taxes may be unlikely to rule him out because of what’s in them. People don’t change their minds because of something like this. People don’t change their minds much at all.

The 2016 election was just a month away four years ago, when The Post unleashed a revelation of its own. The “Access Hollywood” tape didn’t lose its vulgar star the election. Instead, the episode afflicted many with a hopelessness that we still haven’t shaken.

The “nothing matters” mentality, however, is a trap. To see the prophecy’s self-fulfilling bent, look no further than those same comments by Trump on the bus with Billy Bush. Each and every Republican with some public profile stared at a spectrum of options: silence at the far end, the expression of detached disappointment in the middle, the outraged withdrawal of endorsement at the opposite pole. They didn’t want to antagonize an eventual winner; neither did they want to tie themselves to the mast of a ship that just had a cannonball blown into its side.

“I am going to watch his level of contrition over the next few days to determine my level of support,” Sen. Richard Burr (R-N.C.) said at the time. He obviously meant that he would watch the candidate’s level of support to determine his own level of support. That attitude turns all of politics into an elephant eating its own tail: Influential conservatives wished to see how much the disgusting behavior mattered to the rest of the country to decide how much it mattered to them. They either did not consider or did not care that things could work the other way around — that how much the comments mattered to them could play some part in shaping how much they mattered to the rest of the country.

Today, Democrats are in danger of making the same mistake: believing that “nothing matters,” or feeling too silly to keep on screaming “This is bad!” when voters across the country seem to delight in everything devilish the incumbent does.

Yet it’s important to say something anyway, if only for our own sake. When we assume that nothing matters, we project apathy onto others when it comes to the things about which we are the least apathetic. We also cede the debate: We let other people say what matters instead of saying it ourselves — because we’ve defined “matters” as “could make a difference with precisely the right people in precisely the right places to earn enough votes in the electoral college to put someone new in the White House.” But what matters really ought to be a question of values that we aren’t yet ready to give up on.

We also have to say something because we shouldn’t be ready to give up on everybody else, either. The unpaid-taxes story may be more of a threat to Trump than the “Access Hollywood” tape. The president never pretended to be a good guy around women; he has pretended, however, to be a brilliant businessman — and the knowledge of all that debt, and all those losses, could pull out the block that will send his Jenga-tower reputation toppling. It means that the story is more of a threat to Trump’s supporters, too, for whom believing steadfastly in him is an important part of believing inthemselves.

None of us are especially good at admitting that we’ve made mistakes. Our firmest beliefs help us feel moored amid uncertainty. They give us something to grab on to, and we fear that, without them, our broader grip might slip. So we don’t change our minds much, in the end, and that gnaws at our faith that we might change someone else’s.

Yet by saying “nothing matters,’’ we only guarantee that nothing will matter after all.

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