Bill Weld, the 73-year-old former federal prosecutor, two-term governor of Massachusetts, and 2016 vice presidential nominee for the Libertarian Party, announced Monday afternoon on CNN that he is officially running for president in the Republican primary against incumbent Donald Trump.

"I really think if we have six more years of the same stuff we've had out of the White House the last two years, that would be a political tragedy, and I would fear for the republic," Weld told CNN's Jake Tapper. "So I would be ashamed of myself if I didn't raise my hand and run."

In an announcement press release, Weld called himself a "Reagan Republican" who "knows that America can still be that 'shining city on a hill.'"He declared that "it is time to return to the principles of Lincoln—equality, dignity, and opportunity for all," and stressed such non-Trumpian themes as "the rule of law, a free and open press, and America's global leadership." The accompanying campaign video touted among other things Weld's record as a "crime fighter," while leaving his Libertarian adventure of 2016-2018 unmentioned.

As he has since announcing an exploratory committee two months ago, Weld Monday stressed four Trump demerits: The president's deliberate divisiveness, casual disregard for the rule of law, incuriosity about climate change, and fiscal profligacy.

"Donald Trump is not an economic conservative; he doesn't even pretend to be," Weld told Tapper. "And, you know, the country deserves to have some fiscal restraint and conservatism and cutting spending in Washington, D.C. Right now all that really is coming out of Washington is divisiveness, and both parties are responsible for that, but the grandmaster of that is the president himself. I've never seen such bitterness in this country."

One of the main reasons Weld is alone so far in challenging Trump within the GOP is that the market fundamentals he faces are just brutal. The president's approval rating among Republicans has remained between 88 percent and 90 percent for the past five months, according to Gallup. Trump raised more than $30 million in the first quarter, and has consolidated power within the Republican National Committee to a degree frequently characterized as "unprecedented." And the early head-to-head polling, if anything, is worse.

An Emerson nationwide survey released this week has Trump beating Weld by a staggering 70 percentage points. More ominously, the same company this month found a 64-point margin in Weld's home state of Massachusetts. The Weld campaign has a theory that if New Hampshire polls start creeping toward the high 20s, then you're near the "Buchanan benchmark" of 37 percent back during Pitchfork Pat's 1992 primary challenge to then-president George H.W. Bush. You have to squint awfully hard to see such optimism in the numbers, but then, there are still 300 days between now and the New Hampshire primary.

"It is a long shot. But it's certainly less of a long shot than Donald Trump was when he announced and no one thought he was serious," Weld's longtime political adviser Stuart Stevens told the Washington Post. "Tonally, he's going to run a very different campaign. He's not mad at the world. He's not a victim."

One challenge Weld faces among Republican and libertarian-leaning voters alike is his track record of slippery political allegiances and policy positions. In May 2016, during a contentious two-round ballot fight to become the Libertarian Party's vice presidential nominee, Weld was repeatedly asked by the then-dropping out V.P. challenger Alicia Dearn to promise never to "betray" the L.P., as many believe he had done during a botched New York gubernatorial bid in 2006.

"I'm a Libertarian for life," Weld said, trying to make the awkward moment go away without precisely answering the question. Dearn pressed him, saying that "betray," to her, just meant leaving the party, to which Weld declared: "Libertarian for life means not going back to any other party."

To the surprise of many in the L.P., Weld after the 2016 seemed to stay true to that vow, making the state-party convention circuit, endorsing Libertarian candidates, and talking about the need for America's third party to take the 2020 presidential election "seriously." As recently as last the second half of 2018 he was describing his separation from the GOP as being "Free at last," arguing in a Reason debate with Rep. Thomas Massie (R-Ky.) that "If we want a new broom to sweep things clean in Washington, the answer is not the R party or the D party. It is the Libertarian Party," and declaring that it is "our bounded duty to destroy by every means at our disposal" the "two-party monopoly." He switched to the GOP just three months later.

On Monday, Tapper asked the newly re-minted Republican whether he would "run as an independent" in case the primary challenge fell short. Weld's answer contained loopholes you could drive a truck through. "No, no, I don't think so," he said. "But I could not support Donald Trump for president. I'm not saying I would ever endorse a Democrat in this race, but I could not support the president."

As per usual with the orange-haired Brahmin, Weld's announcement Monday produced mixed reactions among libertarians. L.P. National Membership Manager Jess Mears, who worked on the Gary Johnson/Bill Weld campaign in 2016, was magnanimous.

Everyone is all anti-Trump but I don't see anyone other than a libertarian trying to run against him. You go, @GovBillWeld! — Jess Mears (@jess4liberty) April 15, 2019

Fox Business Network's Kennedy, who eviscerated Weld after his infamous last-minute "vouching" for Hillary Clinton on MSNBC's The Rachel Maddow Show, was somewhat less enthused.

Fifth place 2016 finisher Evan McMullin, though, is stoked.

Fantastic news that @GovBillWeld will challenge @realDonaldTrump in the Republican primary. Bill is an honorable Republican who will present a stark contrast to our unfit president and a better way forward for the country. His candidacy deserves strong bipartisan support. https://t.co/teLXmpPKp9 — Evan McMullin (@EvanMcMullin) April 15, 2019

Weld, at a year older than Trump, would be the oldest president in U.S. history. He hasn't won an election in a quarter century, back when Pete Buttigieg was 12. He's a deficit hawk in an era of debt denialism, a proudly elite globalist in a time of ascendant populist nationalism, and a pro-choice, pro-amnesty Republican who backed Barack Obama in 2008 and urged major-party voters in 2016 to choose Hillary Clinton over Trump. There have been more improbable campaigns, but not many.

Reason has interviewed Weld many times, producing the kinds of comments over the past three-plus years you would not normally expect from a competitive Republican politician. "The Libertarian Party is more congenial to me, dogmatically, ideologically, than either of the other two parties," he told me in November 2017, while vowing that he was going to stay in the L.P. "Because the Democratic Party is not fiscally responsible and the Republican Party is not socially tolerant."

We shall soon see whether the GOP is tolerant of internal political competition, or whether Trump will be no more bothered by Weld than Richard Nixon was by Pete McCloskey.