US President Donald Trump leaves a news conference on the novel coronavirus, COVID-19, in the Rose Garden of the White House in Washington, DC on May 11, 2020. Photo: AFP

Democratic-led efforts to impeach US President Donald Trump for a historic second time gained momentum over the weekend, although it looked far from certain whether enough Republicans would back the move with just days left in his term.

Democratic members of the House of Representatives will introduce articles of impeachment on Monday after Trump encouraged his supporters to storm the US Capitol on Wednesday, Representative Ted Lieu said on Twitter.

The California Democrat, who helped draft the charges, said the articles had drawn 190 co-sponsors by Saturday night. As of Saturday afternoon, no Republicans had signed on, Lieu's spokeswoman said.

"We have videos of the speech where [Trump] incites the mob. We have videos of the mob violently attacking the Capitol. This isn't a close call," Lieu tweeted Saturday night.

Trump initially praised his supporters at the Capitol but later condemned their violence in a video. 

The decision to call for calm came at the urging of senior aides, some arguing he could face removal from office or legal liability, sources told Reuters.

Impeachment by the Democratic-led House, equivalent to an indictment, would trigger an unprecedented second trial in the Republican-controlled ­Senate, which cleared him during his first trial over allegations that he threatened US national security.

Two previous presidents were impeached but were also acquitted in the Senate. Richard Nixon resigned in 1974 over the Watergate scandal when it became clear he would be removed.

House Speaker Nancy Pelosi has also asked members to draft legislation aimed at invoking the Constitution's 25th Amendment, which allows for stripping the powers from a president unable to fulfill the duties of the office.

The intensifying effort to oust Trump has drawn scattered support from Republicans, whose party has been divided by the president's actions. 

Democrats have pressed Vice President Mike Pence to invoke the 25th Amendment, but he has opposed the idea, an adviser said.

CNN reported late Saturday that the vice president had not ruled out invoking the 25th Amendment, citing a source close to him, but that some in Pence's team worried any effort to remove Trump could provoke the president to more rash behavior that might put the country at risk.

A Pence spokesman did not immediately respond to a request for comment late on Saturday.

A small but growing number of Republican lawmakers have joined calls for Trump to step down, and several high-ranking administration officials have resigned in protest.