House Intelligence Committee Chairman said Trump poses "a clear and present danger" to democracy.

Top Democrats in the US House of Representatives made a last-minute pitch to Republicans on Sunday for why they should put partisanship aside and vote to impeach President Donald Trump.

Trump is expected to become the third US president to be impeached when the full Democratic-led House votes on the charges, likely this week, setting up a trial in the Republican-controlled Senate.

House Intelligence Committee Chairman Adam Schiff, speaking on ABC's "This Week," warned that Trump poses "a clear and present danger" to democracy.

Schiff and House Judiciary Committee Chairman Jerrold Nadler, speaking on the same show, said lawmakers have a constitutional duty to hold Trump to account for trying to recruit Ukraine to launch a corruption investigation into his Democratic political rival Joe Biden.

Their comments come as the Democratically-controlled US House is poised later this week to vote along party lines on whether to approve two articles of impeachment. The Republican-controlled Senate has shown little appetite for removing Trump from office.

"The misconduct hasn't stopped," Schiff said, noting that Trump has still urged Ukraine, as well as China, to investigate the Biden family, and that Trump's personal attorney Rudy Giuliani is still traveling to Ukraine to conduct a "sham" investigation.

"The threat to our election integrity ... goes on. It's a clear and present danger, I think, to our democracy and not something that we can turn away from simply because the Republicans in the House refuse to do their duty."

Both Democrats used their air time on Sunday to urge the Senate to call witnesses and seek documentary evidence that they said the White House has withheld.

Senate Majority Leader Mitch McConnell has raised the prospect of conducting a short Senate impeachment trial without calling any witnesses.

Nadler on Sunday said such a move would do a disservice to the Senate, whose members will all effectively serve as jurors who will need to weigh the evidence to decide whether Trump should be convicted and removed from office.

"I think the record is overwhelming, but if they don't think there is sufficient evidence on the record, they should demand the testimony of people ... like Secretary of State Pompeo, and Mulvaney, and others- John Bolton- who on the president's instructions have refused to testify," Nadler said.

Some of Trump's staunch Republican defenders in the Senate have already said they believe Trump did nothing wrong, though others have taken a more cautious approach, saying they want to see the evidence first.

"I think we ought to hear what the House impeachment managers have to say, give the President's attorneys an opportunity to make their defense, and then make a decision about whether and to what extent we go forward from there," Republican Senator Pat Toomey said on NBC's "Meet the Press" on Sunday.

He added it would be "extremely inappropriate to put a bullet in this thing immediately" when the House votes to impeach Trump and kicks it over to the Senate for trial.