Democratic prosecutors have charged that Donald Trump's attempt to pressure Ukraine to investigate a political rival is precisely the sort of abuse of power that the nation's founders wanted to guard against when they empowered congress to remove a president from office.

The Democrats also detailed their defence of former vice president Joe Biden's actions regarding Ukraine in anticipation that it will be a major portion of the White House's defence later this week, saying Mr Biden's actions were in line with official US policy at the time and not done to benefit an energy company connected to his son.

But a significant number of Republicans in the Senate remained unmoved and played down the case from House prosecutors, dismissing it as repetitive and unpersuasive as they sought to counter Democrats' narrative at a time when Mr Trump's lawyers must stay silent in the Senate chamber.

House Judiciary Committee chair Jerrold Nadler who led the case for Democrats on Thursday, cited “abuse, betrayal and corruption” as not only the offences that the nation feared the most when they wrote the constitution, but also those alleged abuses at the core of Mr Trump's dealings with Ukraine.

“The president's abuse of power, his betrayal of the national interest, and his corruption of our elections plainly qualify as great and dangerous offences,” Mr Nadler said. “President Trump has made clear in word and deed that he will persist in such conduct if he is not removed from power.”

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During his arguments, Mr Nadler also drew liberally from past comments made by key Trump allies, including senator Lindsey Graham, attorney general William Barr and lawyer Alan Dershowitz, who all argued there doesn't have to be a statutory crime committed to impeach a president.

In one of many video clips played on the Senate floor during the trial, Mr Graham – then serving as a prosecutor in former president Bill Clinton's 1999 impeachment trial – defined “high crimes” more loosely than he does now as one of Mr Trump's staunchest defenders.

“It's just when you start using your office, and you're acting in a way that hurts people, you've committed a high crime,” Mr Graham said on the Senate floor in the clip from 1999. He was not in the chamber Thursday when his quote was played, but later told reporters that the use of his previous comments was “fair game” and recommended that the White House do the same with Democrats when Mr Trump's lawyers begin their defence of the president.

Mr Nadler made a case to the Senate that it would be unreasonable to expect congress to envision all types of potential presidential corruption and pass laws explicitly forbidding it.

“The constitution is not a suicide pact,” Mr Nadler said. “It does not leave us stuck with presidents who abuse their power in unforeseen ways that threaten our security and democracy. Until recently, it did not occur to me that our president would call a foreign leader and demand a sham investigation meant to kneecap his political opponents, all in exchange for releasing vital military aid that the president was already required by law to provide.”

Mr Trump was impeached on 18 December, with Democrats accusing him of withholding nearly $400m (£304m) in military aid and a White House visit coveted by Ukraine in exchange for launching investigations into Mr Biden and his son Hunter, as well as a discredited theory that the eastern European nation interfered in the 2016 election. Mr Trump and his cohort have maintained he did nothing improper when he asked Ukrainian leader Volodymyr Zelensky in a 25 July call to probe the two matters.

In addition to the abuse-of-power charge, Democrats also charged Mr Trump on a second article of impeachment: that he obstructed congressional attempts to investigate the Ukraine matter.

But the second official day of the House impeachment managers' arguments centred on the Democrats' abuse-of-power claims. They will wrap up their case Friday. Mr Trump's team will launch its defence of the president on Saturday, with the key question of whether the Senate will subpoena new witnesses or documents set to be decided around the middle of next week.

The Senate may only come in for a few hours on Saturday morning rather then spending a full day on the trial, in part because Mr Trump did not want much of his defence to be aired on Saturday, when the public may be focused on weekend plans rather than the news, according to a GOP official who like others spoke on the condition of anonymity to discuss the private conversations.

Outside the Senate chamber on Thursday, GOP senators and the president's legal team fanned out across the airwaves to push back against the Democrats' case.

Echoing a comment from several Senate Republicans, Trump attorney Jay Sekulow said “we're hearing the same things over and over” from the House Democrats.

Meanwhile, even if some Republicans didn't outright reject the Democrats' case that there did not need to be an underlying crime for a president to be impeached, most continued to dismiss the overall argument made by the impeachment managers.

“I don't think that anything they've actually alleged – any of the 'abuses' – they have actual evidence for,” senator Josh Hawley said during a break in the trial.

White House officials said on Thursday that West Wing aides and Mr Trump's lawyers will continue to be a frequent presence in Congress over the coming days as the president's team works to defend him. Aggressive public relations and press availabilities are a key part of that strategy, one Republican strategist familiar with the planning said.

“This is going to be more than the president tweeting,” the strategist said, adding that the strategy would include Mr Sekulow addressing the media at microphones set up in congress and representative Mark Meadows “huddling with 100 reporters” in the hallways.

Mr Trump did tweet on Thursday, insisting that Democrats are opposed to a witness deal because they are afraid of what the people called to testify by the GOP would reveal. Democrats have repeatedly said they oppose a witness trade because they believe the witnesses Republicans want are irrelevant to the case.

“The Democrats don't want a Witness Trade because Shifty Schiff, the Biden's, the fake Whistleblower (& his lawyer), the second Whistleblower (who vanished after I released the Transcripts), the so-called 'informer', & many other Democrat disasters, would be a BIG problem for them!” Mr Trump tweeted.

Though the Republicans' top desired witness remains Hunter Biden, Mr Graham told reporters on Thursday that scrutinising the former vice president's son's service on the board of Burisma, a Ukrainian gas company, would be better handled through the “oversight” process than by calling him as a witness during the impeachment trial.

The Bidens played a prominent role in the Democrats' arguments on Thursday, as representative Sylvia Garcia, another impeachment prosecutor, sought to debunk the allegations that Joe Biden did anything nefarious in his dealings with Ukraine during his time as vice president.

“The allegations against Biden are completely groundless,” Ms Garcia said, detailing how the former vice president's efforts to oust the Ukrainian prosecutor general were in line with official US policy and supported by international allies.

But Republican senators said the focus on the Bidens by Democrats made the former vice president and his son fair game for Mr Trump's defence team.

“It means when president Trump's lawyers stand up and present their defence, that they are going to have the opportunity to present the very significant evidence that's supported and still supports a serious investigation into corruption at Burisma and ultimately whether Joe Biden participated in that corruption,” senator Ted Cruz told reporters on Thursday evening.

In 2016, Mr Biden was the Obama administration's point person on Ukraine policy and led a pressure campaign on the Ukrainian government to fire its top prosecutor, Viktor Shokin, whom the administration believed was not doing enough to root out corruption among Ukraine's politicians. Mr Shokin had previously investigated Burisma, but Hunter Biden was not accused of any wrongdoing.

Ms Garcia noted that Mr Trump and Republicans didn't focus on Mr Biden's actions towards Ukraine until 2019, when “Biden became the front-runner for the Democratic nomination and polls showed that he had the largest head-to-head lead against president Trump. That became a problem.”

She also referenced a letter from Republican senators Ron Johnson and Rob Portman, Democratic senator Jeanne Shaheen and other members of the Ukraine Caucus to argue that Mr Biden's desire to see Mr Shokin removed reflected official US policy and was not a sign of personal corruption.

As Ms Garcia spoke, a visibly upset and red-faced Mr Johnson rose from his seat, approached Mr Portman and whispered in his ear. Mr Portman reacted impassively, but his comments did not appear to calm Mr Johnson, who departed the floor for the Republican cloakroom moments later. Mr Johnson, a fierce ally of Mr Trump, said in October that he did not recall signing the letter.

House Intelligence Committee chair Adam Schiff, the lead impeachment prosecutor, addressed the other investigation that Mr Trump wanted Mr Zelensky to launch, saying the president floated a “very specific conspiracy theory” that it was Ukraine, not Russia, that hacked the Democratic National Committee and Hillary Clinton's campaign in 2016 despite there being no evidence to support the claim.

“This theory was brought to you by the Kremlin,” Mr Schiff said. “So we're not talking about generic interference ... what Donald Trump wanted investigated or announced, this completely bogus, Kremlin-pushed conspiracy theory.”

Earlier, Ms Garcia played clips of news interviews from FBI director Christopher Wray and Tom Bossert, a former homeland security aide at the White House, refuting the Ukraine interference theory.

Before the trial officially began on Thursday afternoon, dozens of senators from both parties entered a secure facility in the Senate basement to view a classified document provided by Jennifer Williams, a national security adviser to vice president Mike Pence.

The document was made available under an agreement between Senate majority leader Mitch McConnell and minority leader Charles Schumer; it had been previously submitted to the House Intelligence Committee.

Some senators spent only a few minutes in the facility; others stayed for the better part of an hour. Several Democrats emerged to say they didn't understand why the document had been classified.

“I don't believe it's being withheld from the public for national security reasons. It may be withheld for political security reasons,” said senator Richard Durbin.