WASHINGTON - House Speaker Nancy Pelosi is delaying sending to the Senate articles of impeachment passed by the House, in a tactical move to get the process the Democratic Party wants for US President Donald Trump's trial in the Senate.

Delaying delivering the articles leverages procedure. As lawmakers and constitutional experts argue over procedure, it leaves the President somewhat in limbo over the upcoming holiday week, even as he is sure of an acquittal given that the Republican Party has a majority in the Senate and is backing him.

The Senate will reconvene on Jan 3, Majority Leader Mitch McConnell said on Thursday (Dec 19). Meanwhile, the wrangling will continue.

President Trump wants a swift process. Senator McConnell favours a swift trial as well, hearing from Mr Trump's lawyers and the House impeachment managers - the Democrats' team of prosecutors - before calling just a few witnesses.

The Democrats, however, want a process with more witnesses, with one eye on the potential for testimony from people like former national security adviser John Bolton, and acting White House Chief of Staff Mick Mulvaney.

On Thursday, Ms Pelosi told reporters the articles would be sent and the impeachment managers named, once it was clear what process Senator McConnell will agree to.

"We would like to see a fair process and we will be ready for whatever it is," she said.

The nation's founders had envisioned a "rogue president" but perhaps not a "rogue leader in the Senate" at the same time, she warned.

A meeting on Thursday between Senator McConnell and Senate minority leader Chuck Schumer ended inconclusively. "We remain at an impasse," Senator McConnell said.

Party leaders spent the day trading accusations. The Republican Party slammed Wednesday's impeachment debate and vote in the House as a kangaroo court. The vote to impeach President Trump for abuse of presidential power and obstruction of Congress was almost entirely along partisan lines.

Senator McConnell in a floor speech called the House process "slapdash".

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"It's like the prosecutors are getting cold feet in front of the entire country and second-guessing whether they even want to go to trial," he said.

But Ms Jackie Speier, Democratic Party Congresswoman from San Franscisco, told reporters the delay gives Ms Pelosi leverage.

"If, in fact, they (the Senate) intend to not be an impartial reviewer of the facts, then it becomes a joke. And we're not party to a joke," she said.

President Trump, clearly impatient, tweeted on Thursday: "So after the Democrats gave me no Due Process in the House, no lawyers, no witnesses, no nothing, they now want to tell the Senate how to run their trial."

"Actually, they have zero proof of anything, they will never even show up. They want out. I want an immediate trial!" he added.

"The House Democrats were unable to get even a single vote from the Republicans on their Impeachment Hoax," he tweeted. "The Republicans have never been so united! The Dem's case is so bad that they don't even want to go to trial!"

An analyst close to the Democratic Party, who asked not to be named, told The Straits Times that Ms Pelosi had been rushed into the impeachment, but was now slowing the process down while she strategises on key elements, like the party's team of impeachment managers and the trial procedure.

"This is about making deals on how the trial will be conducted," the analyst said. "She is slowing things down, and she is good at that."