Donald Trump may be the weakest front-runner in recent Republican memory, but he’s the only front-runner the GOP has. And that’s enough to make some sponge-spined members of the party establishment — see: Christie, Chris — decide they’d better jump on the bandwagon before it gets too crowded.

(The same Christie held a press conference to insist he wasn’t being held hostage during a bizarre appearance with Trump on Super Tuesday. That’s always a good sign your endorsement seems sincere.)

But standing before the Trump bandwagon Thursday like the Tank Man at Tiananmen Square — against whom Trump sided, in a 1990 interview with Playboy, instead praising the “strength” Beijing showed in ordering soldiers to kill their countrymen — was the previous GOP standard-bearer, Mitt Romney. Any Republicans ready to hold their nose and fall in line behind Trump might want to listen to his advice.

“Here is what I know,” Romney said toward the end of his speech at the University of Utah. “Donald Trump is a phony, a fraud. His promises are as worthless as a degree from Trump University. He’s playing the American public for suckers: He gets a free ride to the White House, and all we get is a lousy hat.

“His domestic policies would lead to recession. His foreign policies would make America and the world less safe. He has neither the temperament nor the judgment to be president. And his personal qualities would mean that America would cease to be a shining city on a hill.”

That’s quite the condemnation. It’s all true.

Romney had a cautionary message for those who already support Trump:

“I understand the anger Americans feel today. In the past, our presidents have channeled that anger and forged it into resolve, into endurance and high purpose, and into the will to defeat the enemies of freedom. Our anger was transformed into energy directed for good.

“Mr. Trump is directing our anger for less than noble purposes. He creates scapegoats of Muslims and Mexican immigrants. He calls for the use of torture and for killing the innocent children and family members of terrorists. He cheers assaults on protesters. He applauds the prospect of twisting the Constitution to limit First Amendment freedom of the press. This is the very brand of anger that has led other nations into the abyss.”

But despite that warning, Romney didn’t really seem to be speaking to Trump’s supporters. His target audience seemed to be those who haven’t yet fallen, or settled, for Trump.

That’s an important group. Trump is not only weaker, in terms of delegates won, than previous nominees at the same stage of their own races. He’s also won the support of barely more than one-third of voters so far. Nearly two-thirds have opted against him. Even in Georgia, which Trump won handily, fewer than 40 percent of Republicans voted for him this past Tuesday. Exit polls suggest about half of Republicans flat-out don’t want him as their nominee.

So there is nothing inevitable about Trump’s nomination, and plenty of reasons — as Romney articulated — for continuing to actively oppose it.

Above all, those who haven’t yet jumped to Trump might want to think about what kind of party they’ll be left with, if he is allowed to be its face. It certainly is, as Romney said Thursday, like Ronald Reagan before him, a time for choosing.