The anti-Trump onslaught is coming. Perhaps within weeks. Just not necessarily from Republicans.

Almost as soon as Donald Trump is the presumptive GOP nominee — which may be as early as March 15 — Democrats will surely start to churn out their negative ads.

They will attack Trump’s credentials as a tribune of the little guy by focusing on a money-grubbing venture like Trump University, designed to extract as much cash as possible from people who thought they would learn something from the shell of a school.

They will dissect his business record. They will fasten on his failed casinos and the bankruptcies he used to stiff creditors while maintaining a lavish lifestyle.

They will fry him for hypocrisy on immigration by pointing out that Trump Tower was built by illegal Polish immigrants worked to the bone and that, according to news reports, illegal immigrants are helping build his new hotel in Washington.

They will make the cheap threats he throws at anyone who crosses him a character and temperament issue. They will hound him about his unreleased tax returns. And, of course, they will use decades-worth of controversial statements to portray him as racist and sexist.

This will all be in the tradition of the early Democratic ad campaigns that successfully kneecapped Republican nominees in 1996 and 2012 (Bob Dole and Mitt Romney, respectively). A Democratic campaign to disqualify Trump would seek to make his unfavorable rating (already 60 percent with the general public) not merely alarming, but completely radioactive.

How will Trump fare against such ads? Maybe he will prove impervious to all such criticism, or maybe he will wilt under the assault. Who knows?

In this sense, Republicans are outsourcing the vetting of their front-runner to the other party. At this rate, they will make Trump their de facto standard-bearer in a little less than three weeks, never having run him through the paces of the painful testing that is usually inherent to the process.

Yes, Trump has been constantly criticized. But op-eds aren’t the same as attack ads. A Washington Post analysis found that of $215 million super PAC spending so far, only 4 percent has been directed at the man on the cusp of securing the nomination.

A variety of reasons account for the de facto moratorium on sustained Trump attacks to this point: clashing candidate interests; exhaustion after so many donors gave so much to the Jeb Bush super PAC Right to Rise with so little effect; fear of Trump. Democrats won’t be similarly constrained.

The last two viable non-Trump Republican candidates have come up small against the mogul. You would think that the rise of Trump — a seismic political event — would inspire a larger argument about the future of the party, the nature of conservatism or the discontent of blue-collar America, but instead Ted Cruz and Marco Rubio are squabbling over who photoshopped whom.

Cruz has engaged in hostilities with Trump when he can’t possibly avoid them, as in Iowa and South Carolina, but let up in New Hampshire, when it didn’t suit his purposes. Rubio continues to duck and cover, putting tactics (he, of course, wants Cruz out of the race) over leadership.

If Cruz and Rubio aren’t going to consistently attack Trump, at least they should have something to say to his working-class voters who feel they are being left behind and ignored. Yet Cruz and Rubio, loyal sons of their party, do not naturally think in these terms. They know how to have a debate over which of them is more adamantly against gay marriage, but a direct appeal to blue-collar voters on bread-and-butter issues appears outside their comfort zones.

If Trump romps to the nomination by mid-March, non-Trump Republicans will have lost to him in part through a lack of trying. That will never be true of the Democrats, who will gleefully and maliciously do the Trump vetting that the GOP race has, so far, been missing.

Rich Lowry can be reached via e-mail: comments.lowry@nationalreview.com.