Washington, DC - The United States House of Representatives voted on Wednesday to send two articles of impeachment against President Donald Trump to the Senate for trial.

The House's action sets up an historic clash between Congress and the president that now will be fought out in the quasi-judicial and highly political venue of the Senate, where Trump's Republican allies hold a 53-47 majority.

"What is at stake here is the Constitution of the United States. This is what impeachment is about," said House Speaker Nancy Pelosi on Wednesday as she named seven House members who will prosecute the case in the Senate.

"The president violated his oath of office. Undermined our national security. Jeopardised our elections. Tried to use the appropriations process as his private ATM machine to withhold funds in order to advance his personal and political advantage," Pelosi said at a news conference at the US Capitol.

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The Democratic-led House on December 18 impeached Trump for abuse of power over his efforts to get Ukraine to launch an investigation into his political rival, Joe Biden, and obstruction of Congress for refusing to cooperate with the House impeachment investigation. Trump has denied any wrongdoing, labelling the impeachment as a "hoax" and "witch-hunt".

"This is a political impeachment," said House Republican Doug Collins.

"We've shown they don't have the facts. But it doesn't matter because the train is on the tracks and that is the 2020 election for president," Collins said in floor remarks.

Pelosi delayed sending the articles of impeachment to try and pressure Senate Republicans to ensure there would be a fair trial in the Senate.

In the nearly four weeks since the House approved the impeachment articles, several Republican senators have indicated a willingness to entertain Democrats' demands for new witnesses and documents.

"We have always felt a certain urgency about this impeachment given that the president was trying to get foreign help in cheating in the next election," said Adam Schiff, the lead Democrat in the House's impeachment inquiry.

"That time has been very effective in not only bringing new evidence to light - and the evidence was already overwhelming - but also forcing senators to go on the record. Do they want a fair trial, one that's fair to the president and the American people or are they going to participate in a cover-up," Schiff, who will serve as a House manager, said.

House Speaker Nancy Pelosi speaking during a news conference to announce impeachment managers on Capitol Hill in Washington [Susan Walsh/AP Photo]

House impeachment investigators released new material on Tuesday from Lev Parnas, a Ukrainian-American businessman who was working last year with Rudy Giuliani, Trump's personal lawyer, to pressure Ukraine.

Parnas and another Giuliani associate, Igor Fruman, are under indictment by federal prosecutors in New York for campaign finance violations surrounding cash contributions to Trump's political organisation and others. Parnas is cooperating with investigators. The documents included text messages from Parnas's electronic devices.

After the House vote on Wednesday, Pelosi will conduct a formal signing ceremony of the articles of impeachment. The seven House managers will physically carry the documents across the ornate US Capitol building from the House to the Senate.

The Senate trial is scheduled to begin on January 21 and will likely continue for three to four weeks. The trial is expected to move forward with opening arguments before a decision on whether witnesses will be allowed is made.

A small group of Senate Republicans - including Senators Susan Collins, Lisa Murkowski and Mitt Romney - have indicated they may join with Democrats to call new witnesses and testimony in the Senate trial. The Democrats just need four Republicans to join them in calling witnesses.

Democrats want the Senate to call acting Chief of Staff Mick Mulvaney and the former national security adviser, John Bolton, to testify. Bolton, who refused to appear before House investigators, has said he is willing to testify to the Senate if subpoenaed.

Bolton had direct Oval Office conversations with Trump about Ukraine. He called the pressure campaign on Ukraine "a drug deal" that he refused to be part of as national security adviser, according to testimony in the House.