If a third-party presidential candidate were going to have a spoiler effect anywhere in 2020, it would seem to be in Michigan, after Representative Justin Amash, a five-term congressman from Grand Rapids, said this week he was preparing a bid for the Libertarian nomination in the November election.

In 2016, after all, Donald J. Trump won Michigan by just 10,700 votes — and the Libertarian candidate, Gary Johnson, won 172,000 votes in the state. Mr. Amash’s likely entry into the race, as a native son in a state that is again likely to be a battleground, gave some Democrats and Never Trump Republicans overnight heart palpitations.

Nonetheless, strategists from both major parties in Michigan said on Wednesday that this year’s electoral landscape was so fundamentally altered from four years ago that Mr. Amash was not likely to have a major impact in the state, or on the general election as a whole. If Mr. Trump’s surprising victories in Northern industrial states in 2016 were based on high levels of dislike for the Democratic nominee, Hillary Clinton, which depressed Democratic turnout, those appear to be lesser factors this time around as he prepares to face Joseph R. Biden Jr.

Mr. Amash, who was the sole Republican to support impeachment before leaving the party last year to become an independent, said in an interview that he was not worried about easing the president’s path to a second term.