It was completely predictable, and indeed completely logical, that Mr. Amash would not only leave the party as he did last year but — given his vote for Mr. Trump’s impeachment and his obvious, sustained outrage at the administration and the recent direction of the party — that he would look at a third-party challenge as a way of registering his dissatisfaction.

The Libertarian Party’s nominee will almost certainly be on the ballot in all 50 states, as was the case in 2016. The party’s nominee that year, former Gov. Gary Johnson of New Mexico, took 3.3 percent of the presidential vote, winning 9.3 percent in his home state, 5.2 percent in Colorado and about 4 percent in Arizona, all states that both parties see as in play or want to put in play.

If Mr. Amash is nominated, he will become a headache for both parties — one that many Americans feel they have fully earned. Libertarian-minded Trump opponents worry, however, that this headache will afflict Mr. Biden more than Mr. Trump. As much as Mr. Biden is not to their taste on an array of policy issues, there is a sense that he has fewer authoritarian instincts than Mr. Trump and would represent some acceptable form of “normalcy.”

Typical of the responses from many Trump-dubious libertarians following the Amash announcement was this tweet from Julian Sanchez of the Cato Institute: “This is, by a wide margin, the person I’d prefer be president, but … can we not do this right now?”

Maybe Mr. Amash’s percentages will be smaller than Mr. Johnson’s in 2016 — Mr. Trump is now a well-known incumbent, for better or worse, but Mr. Biden seems more likable and electorally viable, for now anyway, than Hillary Clinton ever did for many people. The presumption that Mr. Amash will hurt Mr. Biden as much or even more than he will hurt Mr. Trump looks questionable.

I worked to turn out votes for Mr. Johnson in 2016 to protest the major parties; by my rough estimate, 50 percent of Johnson voters would otherwise have voted for Mrs. Clinton, and 50 percent for Mr. Trump — if they were willing to vote for either at all. There are arguments that third-party candidates hurt Mrs. Clinton, but there are also arguments that Mr. Johnson, at least, helped her, if very slightly.

In 1992, Ross Perot was not on the ballot as a Libertarian, but he is known as one of the most famous spoilers in recent political history, so his effects on the 1992 race remains worth a look. As The Times reported, “If Mr. Perot had not been on the ballot, 38 percent of his voters said, they would have voted for Gov. Bill Clinton, and 38 percent said they would have voted” for George H.W. Bush. In pure Electoral College terms, though, Mr. Perot arguably helped Mr. Clinton a little — in 28 states where Mr. Perot won 20 percent or more of the votes, 17 went for Mr. Clinton and 11 went for Mr. Bush.