Democrats declared that “no president” has ever abused power the way Donald Trump did in his Ukraine dealings, as they opened their second day of arguments Thursday in the historic impeachment trial in the US Senate.

The House of Representatives’ prosecutors pressing their case before sceptical Republican senators are focused on the first article of impeachment, abuse of power, arguing Mr Trump sought a political investigation from Ukraine for his own gain to sway the 2020 election in his favour.

“The president’s conduct is wrong. It is illegal. It is dangerous,” Jerrold Nadler, chairman 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.

Republicans have defended 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.

“No president has ever used his office to compel a foreign nation to help him cheat in our elections,” Mr Nadler said Thursday.

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

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

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

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.

Trump Impeachment
Impeachment manager . Adam Schiff presents an argument in the impeachment trial of President Donald Trump (Dana Verkouteren/AP)

They are one-third of the way through 24 hours of opening arguments.

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

Campaigning in Iowa, 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 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.