Libertarian presidential candidate Justin Amash dismissed the notion that his entry into the race between Donald Trump and Joe Biden could tip the scales in favour of Mr Trump, a popular argument among some political pundits who want to prevent a second term for the president this November.

“We don't know how the additional candidate changes a race, it's too impossible to figure out. There’s too many calculations involved,” Mr Amash, a Michigan congressman who defected from the Republican party after he voted to impeach Mr Trump last year,​ said Sunday in an interview with CNN.

Mr Amash launched an exploratory committee for president last Tuesday.

Political commentators, including former Republican Congressman Joe Walsh, immediately began speculating that Mr Amash could swipe enough independent voters or disaffected Republican voters in swing states such as Michigan and Wisconsin who would otherwise have been inclined to support Mr Biden over Mr Trump, thus increasing Mr Trump's chances at winning a second term.

"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," Mr Walsh wrote in a Washington Post column last week.

Mr Amash's odds of winning the presidency are dim.

As CNN's Jake Tapper pointed out to Mr Amash, recent polling shows that only roughly 10 per cent of US adults say they would not be satisfied with choosing either Mr Trump or Mr Biden.

Still, Mr Amash believes he can topple the American two-party system by carving out a lane among independents like himself and people disaffected with the Democratic and Republican parties.

"I'm gonna win the election," Mr Amash said.

"When you look at a lot of different polls out there, you'll see that a good portion of the country, probably a plurality is pretty independent, and they are looking for another choice," he said.

No third-party candidate has won a modern US presidential election, although experts believe some have swung elections by siphoning votes that otherwise would have gone to the losing major-party candidate.

In the 2000 election, George W. Bush defeated Al Gore by 537 votes in Florida, where Independent Ralph Nader received 97,421 votes. Exit polls at the time showed more Nader voters in Florida would have voted for Mr Gore than for Mr Bush.

Mr Amash believes he can be more than just a spoiler, though.

“For too long we've had the same system, where these two parties go at each other and Washington is totally dysfunctional. That’s why I left the Republican party,” he said on Sunday.

“We need someone who is going to come in as president, respect our constitution, defend our rights and fix our representative system of government so people will actually feel represented at home,” he said.