Because even his opponents know there can be no gainsaying his ability to serve as president; or his talent for appointing competent deputies; or his mastery of the mechanics of government; or his overwhelmingly successful tenure as New York mayor; or his understanding of business and the economy; or his immediate credibility on the world stage; or his sobriety of judgment or general probity of character. Mike Bloomberg has donated more money — at least $6 billion — than Donald Trump has ever had.

Because the voters who will matter in the election — that is, those who live neither in deep blue nor deep red states — want a centrist. Democrats in Michigan, Wisconsin, Pennsylvania, North Carolina, Arizona and Florida tell pollsters they want “a Democratic nominee who is more moderate than most Democrats,” and that they prefer “one who would bridge the partisan divide,” according to a report Friday from my colleagues Jonathan Martin and Katie Glueck.

Oh, and because Bloomberg is what Trump only pretends to be: a bona fide billionaire and proven entrepreneur. In 2012, the Romney campaign tried to create a contest between “makers and takers.” A Bloomberg-Trump contest would be one between a maker and a faker.

All this should terrify the Trump campaign. Yes, Bloomberg has some weaknesses as a candidate. These include age (77), a less-than-charismatic speaking style, a reputation as a scold who wants to take away your supersized fizzy drink, his (Trumpian) reluctance to release his full tax returns, and the fact that he’s a technocratic master-of-the-universe in an era of populist demagoguery.

None of these should be deal-breakers for any persuadable voter. The real question is whether Democrats can quickly get the message that, contrary to what many of them have supposed, the election is not theirs to lose. A candidate who flubs his lines against Trump will yield the same disastrous result as one who sounds like the Party Line.

Of course, the Democrats could always get lucky. The recession might arrive by Easter . A convincing process of, and vote for, impeachment in the House could move a few Senate Republicans to vote for conviction. Some other major Trumpian scandal might come to light.

But if trouncing Donald Trump is essential to the preservation of liberal democracy, then it won’t do to cross fingers and hope he stumbles. A Bloomberg candidacy would be a gift to Democrats, the country, and the world. Sneer at it at your peril.