Democrats in the United States have declared "no president" has ever abused power the way Donald Trump did in his Ukraine dealings.

The comments come as Democrats open their second day of arguments on Thursday in the historic impeachment trial of President Trump.

The House of Representatives' prosecutors are pressing their case before a group of sceptical Republican senators, and have begun by focusing on the first article of impeachment - abuse of power.

They are arguing Mr Trump sought a political investigation from Ukraine for his own gain to sway the 2020 election in his favour.

Jerrold Nadler told the impeachment trial Mr Trump's 'conduct is wrong'. Credit: Senate Television/AP

"The President’s conduct is wrong.

"It is illegal.

"It is dangerous," Jerrold Nadler, Chair of the House Judiciary Committee, told the senators.

House Democrats impeached Trump last month, arguing he abused his office by asking Ukraine to investigate political rival Joe Biden while withholding crucial military aid.

They also charged him with obstructing Congress by refusing to turn over documents or allow officials to testify in the House probe.

  • Donald Trump's impeachment trial explained

On the other side, Republicans have defended Mr Trump's actions as appropriate and cast the process as a politically motivated effort to weaken the President in the midst of his re-election campaign.

The Democrats' challenge is clear as they try to convince not just fidgety senators but an American public divided over the Republican President in an election year.

Mr Nadler told senators on Thursday: "No President has ever used his office to compel a foreign nation to help him cheat in our elections.

"Prior presidents would be shocked to the core by such conduct, and rightly so."

Impeachment manager Adam Schiff presents an argument in the impeachment trial. Credit: Dana Verkouteren/AP

Republican senators, who hold a majority in the chamber and will vote on Mr Trump’s conviction or acquittal, exhibited no shock at the comments.

The top Senate Democrat, Chuck Schumer, acknowledged on Thursday that many senators "really don’t want to be here".

Only if two-thirds of the senate, 67 senators, vote to convict the president will he be removed from office. As it stands the senate is made up of 53 Republicans and 45 Democrats - with 2 independents who both caucus with the Democrats.

President Trump has branded formal impeachment charges 'a complete hoax' Credit: PA

Once reluctant to take on impeachment during an election year, Democrats are now marching toward a decision by the Senate that the American public also will judge.

Meanwhile Donald Trump has continued to express his dissatisfaction with the impeachment trial on Twitter.

Mr Trump blasted the proceedings in a Thursday morning tweet, declaring them the "Most unfair & corrupt hearing in Congressional history!"

Back in the impeachment trial, Democrats are one-third of the way through 24 hours of opening arguments.

Campaigning in Iowa to be the Democrats' presidential candidate, Mr Biden said: "People ask the question: ‘Isn’t the president going to be stronger and harder to beat if he survives this?’

"Yes, probably.

"But Congress has no choice."

He said senators must cast their votes and "live with that in history".

Each side has up to three days to present its case.

After the House prosecutors finish, likely on Friday, the President’s lawyers will have as much as 24 hours.

It is unclear how much time they will actually take, but Mr Trump's team promises not only to defend the president but to take apart the Democrats' case.

The Senate is expected to take only Sunday off and push into next week.