Donald Trump has broken his Twitter record by posting more than 140 times in less than 18 hours, as senators consider whether to make him the first president to be impeached from office .

By 5.30pm Washington time, Mr Trump had tweeted or retweeted once every seven minutes since midnight.

Many of the tweets involved the trial, where impeachment managers from the House of Representatives are currently putting forward the case to the Senate .

"NO PRESSURE!" wrote Mr Trump in one message.

Eight House Democrats are making the case for impeaching Mr Trump, while House Republicans and Mr Trump's lawyers are defending him.

"President Trump solicited foreign interference in our democratic elections, abusing the power of his office to seek help from abroad to improve his re-election prospects at home," Democrat Representative Adam Schiff said.

"President Trump withheld hundreds of millions of dollars in military aid to a strategic partner at war with Russia to secure foreign help with his re-election.

"In other words, to cheat."

Donald Trump could be the first president in history to be removed from office. (AP)

Via Twitter, Mr Trump described Mr Schiff as a "con job" and "corrupt".

Yesterday the trial stretched on for more than 12 hours, with hearings and debates extending well past midnight, Washington time.

Democrats have been pushing for witnesses to be allowed to speak at the trial, but Republicans, who hold power in the Senate, have voted unanimously to prevent such testimony.

But Democrats are skirting the rules by allowing video footage of Mr Trump to play for the senators.

Senate Minority Leader Chuck Schumer walks away from a throng of reporters in the Capitol. (AP)

In one, Mr Trump told a television reporter that he would accept damaging information about an opponent from Russia.

Receiving anything of value from a foreign entity for political campaign purposes is a crime.

Obstruction of justice

Complicating the trial is the refusal of the White House to turn over documents subpoenaed by Congress.

The refusal is being used as evidence in the obstruction of justice charge Mr Trump is facing.

House impeachment manager Jason Crow presents redacted documents to the Senate. (AP)

Speaking from Switzerland today, Mr Trump bragged about withholding the evidence.

"Honestly, we have all the material. They don't have the material," he said.

The White House has been stonewalling subpoenas after a call summary of a phone call between Mr Trump and the Ukrainian president galvanised the impeachment case.

The summary showed Mr Trump pressuring the Ukrainian president to announce an investigation of his chief Democratic rival Joe Biden .

Former Vice President Joe Biden is one of the frontrunners in the presidential race. (AP)

The White House also blocked US$500 million in congressional approved military aid for Ukraine .

The challenge for Democrats is not to prove Mr Trump committed a crime, but to convince Republican senators that they should vote to impeach him.

While a majority of Americans favour removing Mr Trump from office, according to three recent polls, those that oppose make up the Republican voter base.