Mitt Romney has done something that no Republican presidential nominee has done during the modern primary process: denounced the man who is the frontrunner to succeed him as the party's standard-bearer.

The speech was a doozy. He went after Trump's personal character, his finances, his positions on issues ranging from trade to Iraq. "Dishonesty is Trump's hallmark," Romney said. The audience cheered his list of indictments, as did anti-Trump Republicans on social media who were watching his remarks on television.

Romney is a nice man. He was a reasonably good governor in Massachusetts. He is the worst possible spokesman for Republicans against Trump.

First, he is the losing Republican from 2012. I mention "losing" — the thing Romney did in the Electoral College, the popular vote, and nearly all swing states when he ran against President Barack Obama four years ago — because Trump is sure to bring it up. In fact, he already has. "Mitt is a failed candidate," Trump quickly responded Thursday. "He failed horribly. That was a race that absolutely should have been won."

A big part of Trump's shtick is that he was a good soldier for the Republican establishment when Romney and John McCain were the party's nominees. They failed and now he is taking matters into his own hands. (It's worth recalling Romney did more than any other establishment Republican to help elevate Trump by appearing with the billionaire to accept his endorsement four years ago.)

For Trump, this serves two purposes. It helps him bond with Republicans who are angry about their party's failure to beat Obama in legislative battles or elections. And it is a conversion story of sorts to explain why he went from being a fairly conventional moderate Northeastern Republican businessman — albeit one who has long been consumed by themes of national decline and foreigners disrespecting America — to a Tea Party populist.

The type of Republican who is drawn to Trump believes the party leadership is more interested in fighting the rank and file, or fighting people who are perceived as being too conservative, than fighting the Democrats.

Romney going after Trump with much more relish and cleverness than he ever displayed in his rote attacks on Obama doesn't really do anything to allay those concerns. In fact, it basically makes Trump's case for him.

While anti-Trump Republicans are hungry for leadership and will welcome it from almost any source, Romney subtly undermines the conservative case against Trump in other ways. That case revolves around the fact that Trump isn't especially conservative by mainstream movement standards.

A Republican leader whose conservatism was also suspect by mainstream movement standards, even if to a much lesser extent, isn't the best person to press that argument against Trump. If anything, it reminds the Trump-inclined that conservatives who have denounced their guy once accepted or even endorsed Romney while other more conservative alternatives were still available.

Another conservative criticism of Trump is that even on the issues where he now formally agrees with conservatives, he has a history of flip-flopping. Again, this isn't a subject on which Romney is the best spokesman. Romney's list of flip-flops is certainly less comprehensive than Trump's. But a conservative activist used to follow Romney around dressed as a dolphin to represent Romney's predilection for position changes. It's a reputation he has had to deal with for a long time.

One of the issues on which Trump has 1. not been very conservative and 2. flip-flopped is health care. Trump has in the not too distant past spoken favorably of single payer. You see the problem, yes? Romney signed and helped design the Massachusetts health care law that was a model for ObamaCare.

RomneyCare is to the right of single payer, yes. So is ObamaCare. Do opponents of government-run health care want to quibble about these technical details in 2016? Wasn't the idea that conservatives should support whatever is incrementally to the right of the Democrats' current health care plan, even if it involved individual mandates and subsidies, what got us into the whole RomneyCare/ObamaCare mess in the first place?

Finally, the argument has always been that once Republicans like Romney go on a winning streak in the primaries, there is nothing we can do to deny them the nomination. If Trump can be denied the nomination after winning two-thirds of the states that have voted this year, it raises questions about the inevitability of past frontrunners.

Yes, Romney has more character and integrity than Trump. And yes, the David Duke/Ku Klux Klan business adds some moral gravity to the conservative case against Trump beyond disagreements on trade policy and carried interest.

That still doesn't mean Romney is the best person to rally Republicans against Trump. The best people to do that are conservatives who actually understand Trump's appeal to so many GOP voters.

On some level, Romney must know that the best he can do is prevent the perception that the party is coalescing around Trump. And maybe that's enough.

But if Romney thought he could really help a non-Trump candidate win the nomination, he would probably endorse one. The fact that his endorsement would be as likely to hurt a Marco Rubio or John Kasich as help them (Ted Cruz could probably survive it) is the best indication that if you are looking for an anti-Trump, the Mitt just doesn't fit.