WASHINGTON (Reuters) - Justin Amash, the first Republican in the U.S. Congress to say openly that President Donald Trump has committed impeachable offenses, on Monday fired back at critics, including Trump.

FILE PHOTO: U.S. Rep. Justin Amash (R-MI) speaks at the Liberty Political Action Conference (LPAC ) in Chantilly, Virginia September 19, 2013. REUTERS/Kevin Lamarque/File Photo

Standing behind his earlier remarks, Amash issued a string of tweets that challenged some of the most common arguments of those who defend Trump over Special Counsel Robert Mueller’s probe of Russian meddling in the 2016 U.S. election.

The new Amash tweets followed his earlier remarks on Twitter on Saturday, when he said that the Mueller report on Russia showed that Trump, a fellow Republican, had obstructed justice.

“President Trump has engaged in impeachable conduct,” the Michigan conservative said then, drawing a broadside from Trump.

In his usual caustic style, the president on Sunday tweeted that Amash was “a total lightweight” and “a loser.”

In addition, in a case of swift political retribution, Amash drew an election challenge from within his own party on Monday when Jim Lower, a Michigan state legislator who described himself as “pro-Trump,” said he would challenge Amash in the 2020 Republican primary, the Detroit Free Press reported.

Amash in his latest tweets said that people who say Trump could not have intended to illegally obstruct Mueller’s investigation relied on several falsehoods, including a claim that there were no underlying crimes.

“In fact, there were many crimes revealed by the investigation, some of which were charged, and some of which were not,” Amash wrote on Twitter.

Mueller’s investigation led to criminal charges against 34 people, including Trump’s first national security adviser, Michael Flynn, who agreed to cooperate with the probe.

Amash also said that bringing an obstruction of justice case did not, as some of Trump’s backers have argued, require the prosecution of an underlying crime.

Further, he said, “high crimes and misdemeanors,” the U.S. Constitution’s standard for removing a president from office, does not require corresponding statutory charges. “The context implies conduct that violates the public trust,” Amash said.

No U.S. president has ever been removed from office as a direct result of the U.S. Constitution’s impeachment process.

Democrats have debated for months whether to start proceedings to remove Trump from office, but no Republican in Congress, other than Amash, has called Trump’s conduct impeachable. While Amash’s remarks made calls in Congress for Trump’s removal bipartisan, there were no signs late on Monday of other Republicans following his lead.

A long-time Trump critic, Amash is part of the House Freedom Caucus, a conservative House of Representatives faction whose members normally defend Trump. Amash has also signaled he would consider running as a libertarian against Trump in 2020.

Representative Mark Meadows, the leader of the Freedom Caucus, told reporters Monday that he and other members of the group do not agree with Amash’s impeachment remarks, calling them “poorly informed” and a “faulty analysis.” But, he said he still expected Amash to be re-elected again to Congress next year.

Meadows said Amash’s conclusion about impeachment was an outlier among Republicans. “This is not the crack or the floodgates opening up because of this one person,” Meadows said. “In fact it’ll have no impact on any other Republican member in terms of their position in support of the president.”

Amash has been in Congress since 2011 and has faced only one serious primary challenge since then. He beat that opponent by nearly 15 points in 2014.

Michigan voters helped put Trump in the White House in 2016 by a slim margin. Democrats gained ground in 2018’s congressional and state elections, making the Midwestern state a key political battleground in 2020.