US congressman Justin Amash abandoned his third-party run for president Saturday, blaming the impossibility of campaigning during the COVID-19 pandemic.

The former Republican, who left the party amid frustrations over the leadership of US President Donald Trump, had announced he was preparing to run less than three weeks ago, on April 28.

"After much reflection, I've concluded that circumstances don't lend themselves to my success as a candidate for president this year, and therefore I will not be a candidate," he tweeted. 

Social distancing, political polarization and the impact of the economic crisis on fundraising were among the reasons he cited for calling off his run. 

"I continue to believe that a candidate from outside the old parties, offering a vision of government grounded in liberty and equality, can break through in the right environment. But this environment presents extraordinary challenges," he said.

Amash, 40, left the Republican Party in the summer of 2019.

As an independent in Congress, he voted to impeach Trump. 

In April he announced he had formed an exploratory committee in preparation for a run as the nominee of the Libertarian party, which promotes civil liberties and limited government. 

"Let's do this," the Michigan lawmaker said at the time. 

He had presented himself as an alternative to Trump and the presumptive Democratic nominee, Joe Biden. 

In the 2016 presidential election, Libertarian Party candidate Gary Johnson received nearly 4.5 million votes, or just over three percent of ballots cast.