Donald Trump and first lady Melania Trump arrive at Dover Air Force Base to attend a ceremony in Dover Air Force Base, Delware, on Thursday. Evan Vucci / AP

Donald Trump's conduct with Ukraine was far worse than the behaviour that led to Richard Nixon's downfall, the head of the congressional impeachment probe said on Thursday as he summed up the case for the US president's removal.

Lawmakers investigating whether Trump abused his office by bullying Ukraine into assisting his reelection efforts have spent three days hearing the most explosive witness testimony since the scandal broke.

Democrats accuse Trump of demanding Ukraine's assistance in a July 25 phone call and via a circle of accomplices, mainly senior figures in his administration.

He stalled almost $400 million in military aid and a White House visit for new Ukrainian president Volodymyr Zelensky, allegedly to coerce Kiev − which is at war with Russia − into helping.

The plot would have worked, say Democrats, if not for a whistleblower whose complaint about the July call reached Congress on September 9, ensuring the aid was handed over two days later.

"What we've seen here is far more serious than a third-rate burglary of the Democratic headquarters," said House Intelligence Committee chairman Adam Schiff, referring to the infamous 1972 Watergate break-in that Nixon ordered, eventually leading to his resignation.

"What we're talking about here is the withholding of recognition in that White House meeting (and) the withholding of military aid to an ally at war. This is beyond anything Nixon did."

Schiff spoke at the close of marathon testimony by nine witnesses over three days − on top of two days of hearings last week − that produced an avalanche of corroboration for the allegations against Trump.

Democrats are expected to prepare formal articles accusing Trump of abusing his presidential powers, bribery and obstruction of justice.

The investigation threatens to make him the third US president to be impeached, although the Republican-controlled Senate would need to convict him to remove him from office.

In bombshell testimony on Wednesday, Washington's envoy to the European Union testified that Trump tied the White House meeting to the investigation of Democratic 2020 frontrunner Joe Biden.

Ambassador Gordon Sondland told the panel Trump ordered him and other diplomats to answer to the president's personal lawyer Rudy Giuliani, a private citizen with no public office, on Ukraine policy.

The former Soviet state, Democrats have noted, is not in the European Union.

Giuliani's demands to Ukraine for investigations, Sondland said, "were a quid pro quo for arranging a White House visit for President Zelensky."

Agence France-Presse