NEW YORK — It takes either a brave man or a delusional one to mount a primary challenge against a president with 90 percent approval in his own party. That’s what former Massachusetts governor William Weld has done in taking on President Trump. But when, during an interview in my office last week, I suggested that he might be on a kamikaze mission, he airily waved those concerns away.

“Well, I’m spending a lot of time in New Hampshire, and I’m seeing Trump fading and we’re coming up,” he told me. “I started to sense that in August when more people were coming up to me in the street and saying, ‘Hey, Governor. Great to see you, saw you on TV, heard you at this and such meeting,’ as opposed to me going over to them saying, ‘Hi, I’m Bill Weld, I’m running for president as a Republican against Mr. Trump.’ So the president did not have a good August, and then he had quite a bad September. . . . I’m still way behind, but at least it’s not 98 to 2!”

A congenital optimist in the mold of his hero Theodore Roosevelt, Weld hopes to rival the 42 percent that Eugene McCarthy received against President Lyndon B. Johnson in New Hampshire in 1968 and the 37.5 percent that Pat Buchanan received against President George H.W. Bush in 1992. There is no sign that either Weld — or former Rep. Joe Walsh, the other GOP primary challenger — is anywhere close to that level. (Trump’s worst poll in New Hampshire, from mid-October, had Weld at 14 percent and Walsh at 5 percent.) But Weld is hoping that Democrats reregistered as undeclareds — voters with no party affiliation can vote in either party’s primary in New Hampshire — in order to strike an early blow against Trump.

He is also optimistic that the impeachment proceedings will damage Trump. Another primary challenger, former congressman Mark Sanford, has already pulled out, Weld told me, because he worried “that the impeachment is going to suck all the oxygen out of the room so no one will hear him talk about the deficit.” Weld, by contrast, says “that impeachment could put some oxygen into the room.”

This faith is hard to sustain when even retiring Republicans such as Reps. Will Hurd (Tex.) and Mac Thornberry (Tex.) continue to defend Trump’s conduct as “inappropriate” but not “impeachable.” But Weld told me: “I’ve been a litigator for a long time, and I’ve seen cases that looked a lot different at the end than they did at the beginning because the weight of pretty much incontrovertible sworn testimony and documentary evidence can create hard facts. And as John Adams said, ‘Facts are stubborn things.’ ”

Of course, Adams spoke in the days before Fox News made facts into malleable things. But Weld points out that impeachment and removal are already favored by roughly 50 percent of the public. If that number goes up to 60 percent, he argues, Republican senators could begin to ditch Trump as a political liability.

Weld fears that, despite the devastating revelations about Trump’s dirty dealings in Ukraine, the president could still win reelection against a far-left opponent. “He beats [Elizabeth] Warren any day,” he said, “because she is completely dug into the position that the private sector doesn’t create any jobs, they’re all created by government.” (He is referring to Warren’s version of President Barack Obama’s “you didn’t build that” refrain to suggest that all job creation is the result of public sector investments.) “My nightmare is Warren loses to Trump if Trump is the nominee, which is why I’m trying to do something about it.”

It is too bad that Weld is not getting more traction, because I am convinced he would be a better president not only than Trump (a very low standard) but also than any of the Democratic front-runners. A quintessential moderate — a fiscal conservative and social liberal — Weld is also, as he told me, “a proven reacher-across-the-aisle. . . . The atmosphere in Washington would change radically on Day One if I were to replace Mr. Trump.” He has the right temperament for the office: “I’ve always been very comfortable in my own skin, and I’m a calm and relaxed person.” The contrast with the dyspeptic incumbent is so obvious that he doesn’t even have to draw it.

The problem is that Weld hasn’t won an election since 1994, and only recently returned to the Republican Party after having been the Libertarian vice presidential nominee in 2016. But he is an attractive, quirky figure who might — just might — catch on in the first-in-the-nation primary. If billionaires such as Tom Steyer and Michael Bloomberg really want to stop Trump, they wouldn’t be running themselves; they would be devoting big bucks to funding Weld and Walsh to soften up Trump in the primaries (while also funding moderate candidates in the Democratic field).

Remember that neither Lyndon Johnson nor George H.W. Bush survived weaker-than-expected showings in New Hampshire. Trump may be unstoppable, but he is more likely to suffer a serious setback among the flinty voters of the Granite State than among the quivering mass of Republican weaklings in Washington.

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