Former Vice-President Joe Biden says impeachment proceedings should begin against Donald Trump if he does not comply with an inquiry into a call he made with the Ukrainian president.
The Democratic presidential front-runner is the latest top party member to join impeachment calls.
Democrats accuse President Trump of improperly pressuring the Ukrainian president to undermine Mr Biden.
No US president has ever been removed from office by impeachment.
Impeaching Mr Trump "would be a tragedy", Mr Biden said. "But a tragedy of his making."
President Donald Trump has acknowledged withholding US aid to Ukraine and pressing his Ukrainian counterpart to investigate his would-be White House challenger Joe Biden.
But he denied using the funds as political leverage, insisting he only wanted Europe to step up assistance to the Eastern European country.
House Democrats are meeting shortly and US media report that the most senior Democrat, House Speaker Nancy Pelosi, will launch a formal impeachment inquiry.
What exactly did President Trump say?
On Tuesday, Mr Trump said he only froze military aid to Ukraine because he wanted European countries to contribute funds, too.
"We're putting up the bulk of the money, and I'm asking why is that?" he said, adding: "What I want, and I insist on it, is that Europe has to put up money for Ukraine also."
The Republican president also acknowledged pressuring newly elected Ukrainian President Volodymyr Zelensky during a phone call on 25 July to investigate US Democratic presidential frontrunner Joe Biden.
"That call was perfect, it couldn't have been nicer," said Mr Trump, who is up for election next year. "There was no pressure put on them whatsoever.
"But there was pressure put on with respect to Joe Biden. What Joe Biden did for his son, that's something they should be looking at."
Mr Trump later tweeted he had authorised the release "of the complete, fully declassified transcript of my phone conversation with President Zelensky of Ukraine".
"You will see it was a very friendly and totally appropriate call," he wrote.
Mr Trump and his conservative allies have pointed out that Joe Biden, while US vice-president, threatened in 2016 to withhold aid to Ukraine unless it fired a top prosecutor whose office had been investigating a natural gas company where Hunter Biden was a board member.
Other Western officials had called for the same prosecutor to be removed on the grounds that he was soft on corruption. Ukraine's current prosecutor, Yuriy Lutsenko, told Bloomberg News in May he had no evidence of wrongdoing by Mr Biden or his son.
Mr Trump's latest remarks came after it emerged that days before his phone call with Mr Zelensky, the US president had instructed his acting chief of staff, Mick Mulvaney, to withhold nearly $400m (£320m) in military aid for Ukraine.
Congressional Democrats have accused Mr Trump of trying to enlist a foreign power to smear a domestic opponent.
They are demanding a transcript of the phone call, which the White House has declined to release.
The controversy came to light after a US intelligence whistleblower filed a complaint with an internal watchdog about Mr Trump's conversation with the Ukrainian president.
What's the latest on impeachment?
The House of Representatives' Democratic leader, Speaker Nancy Pelosi, is holding a closed-door meeting with party members on Tuesday to consider impeachment.
She has so far resisted calls among her liberal rank-and-file to attempt to remove the Republican president from office as such a move lacks the support of most members of Congress and the American public.
Nearly a dozen Democrats have come out in favour of impeachment over the last week since the Ukraine phone call controversy came to light.
More than 145 House Democrats now back such a move - more than half of the party's 235 members in the lower chamber of Congress. The Senate, though, remains in Republican hands and is unlikely to back impeachment.
How is Biden responding?
In a statement on Tuesday afternoon, Mr Biden called the president's accusations "baseless and without merit".
"I can take political attacks, they'll come and they'll go," Mr Biden said. But, "if we allow a president to get away with shredding the United States Constitution, that will last forever."
"It's an abuse of power, it undermines our national security...and it strikes at the heart of the sworn responsibility of the president."