Could a third man be a spoiler in race for the White House?

The first major third-party candidate has emerged in the contest between President Donald Trump and presumptive Democratic nominee Joe Biden.

Michigan congressman Justin Amash wants to seek the White House as a Libertarian after switching from Republican to independent last July 4 and voting in favour of Mr Trump’s impeachment.

That has annoyed conservative “Never Trumpers” whose ranks have dwindled since 2016 but who nonetheless fear that any serious alternative could shift the support of wary Republicans away from Mr Biden and help ensure that the president secures a second term.

Having Mr Amash in the race could also force Mr Biden to place extra emphasis on key battleground state Michigan.

Mr Amash shrugged off concerns that his candidacy could benefit either Mr Trump or Mr Biden.

“I hear from people who support Joe Biden that I’m helping Trump,” Mr Amash said.

“I firmly believe that the positions I hold and the principles I espouse are ones that reflect a larger portion of the electorate than the number of people supporting either Donald Trump or Joe Biden.”

Former Illinois representative Joe Walsh, who challenged Mr Trump in the Republican 2020 primary, thinks that, by running, Mr Amash will ultimately help Mr Trump — whom he derided as an “authoritarian con man”.

“He can siphon enough votes from the Democratic nominee, Joe Biden, to hand the election to Trump,” Mr Walsh wrote in a Washington Post op-ed.

“If Amash gets the Libertarian nomination and stays in until the end, he could wind up going in the books as the guy who voted to impeach Trump one year, then tipped the election to him 11 months later.”

Former New Mexico governor Gary Johnson ran on the Libertarian ticket the last two presidential cycles and got nearly 4.5 million votes in 2016, though he has said he will not run again.

But attention four years ago focused more on Green Party nominee Jill Stein, who won more votes in Michigan and Wisconsin than Mr Trump’s small margin of victory in both states.

That led some Democrats to blame Ms Stein for helping catapult Mr Trump to the White House, an accusation her party rejected — and no one knows for sure if Green voters might have sided with Democrat Hillary Clinton had they known the race would be as close as it turned out to be in their states.

“I think Amash would make a wonderful candidate,” the president tweeted.

“He almost always votes for the Do Nothing Dems anyway.

“I like him even more than Jill Stein!”

Third-party candidates can make a decisive impact on US elections.