Us President Donald Trump has hit out at the first Republican congressman to call his behaviour impeachable, as Democrats warned that Mr Trump's stonewalling of congressional probes is strengthening the case for an impeachment inquiry.

Mr Trump called Republican Representative Justin Amash "a total lightweight" and "a loser" on Twitter, after the Michigan conservative said the Mueller Report showed that the Republican president "engaged in specific actions and a pattern of behaviour that meet the threshold for impeachment."

Mr Amash's criticism made calls in the US Congress for Mr Trump's impeachment bipartisan, though just barely, with most Republicans still standing by the president at a time of economic growth, turbulent markets and global trade tensions.

Saying most politicians have not read it, Mr Amash cited Special Counsel Robert Mueller's 448-page report on Russian meddling in Mr Trump's favour in the 2016 US election.

On Twitter yesterday he said the report showed Mr Trump had obstructed justice and added, "President Trump has engaged in impeachable conduct."

A frequent Trump critic, Mr Amash is a part of the House Freedom Caucus, a conservative faction.

He has also signalled he would consider running as a libertarian against Mr Trump in 2020.

Counter-punching in his usual style, Mr Trump tweeted: "Never a fan of @justinamash, a total lightweight who opposes me and some of our great Republican ideas and policies just for the sake of getting his name out there through controversy ... Justin is a loser who sadly plays right into our opponents hands!"

Mr Trump will have a chance to make his case to supporters tomorrow at a rally planned for Montoursville, Pennyslvania.

Mr Amash's comments echoed the conclusions of many Democrats.

House of Representatives Speaker Nancy Pelosi has said that Mr Trump was moving closer to impeachment with his stonewalling of numerous congressional investigations of him and his presidency.

Still, Democrats are divided about impeachment. With 2020 election campaigns heating up, Ms Pelosi said impeachment proceedings would be "divisive" for the country.

No US president has ever been removed from office as a direct result of the US Constitution's impeachment process.

The House of Representatives has impeached two presidents. Both were acquitted by the Senate.

The Mueller report, now at the centre of an escalating oversight battle between Mr Trump and House Democrats, detailed extensive contacts between his 2016 campaign and Russia, but did not find that there was a conspiracy with Moscow.

The report also described actions Mr Trump took to try to impede Mr Mueller's investigation, but made no formal finding on the question of obstruction, leaving the matter to Congress.

Mr Amash also said on Twitter that Attorney General William Barr, who was appointed by Mr Trump, "deliberately misrepresented" the Mueller Report when he oversaw the roll-out of a redacted version.