WASHINGTON (DPA) - US President Donald Trump's personal lawyer Rudy Giuliani says Democratic lawmakers and the media are trying "destroy" him to cover up unsubstantiated allegations against former President Barack Obama's administration.

Giuliani is a central character in the political firestorm and impeachment proceeding into Trump's dealing with Ukraine and alleged attempts to unduly pressure the Ukrainian government for his own political gain.

"I knew the Swamp Media and corrupt Dems would try to destroy me to continue their multi-year cover-up of the scandal at the highest levels of the Obama administration!" Giuliani said on Twitter on Saturday (Sept 28).

Giuliani is repeatedly mentioned in a call between Trump and Ukrainian President Volodymyr Zelenskiy in which Trump asked Zelenskiy to investigate his political rival and former vice-president Joe Biden.

The call was at the centre of a whistleblower complaint, which alleged Trump tried "to solicit interference from a foreign country in the 2020 US election."

There was also concern Trump was withholding military aid to Ukraine to get his wishes.

Giuliani has accused Biden of wrongdoing for pressuring Ukraine to fire a prosecutor while his son Hunter was on the board of a natural gas company under scrutiny by the prosecutor.

No evidence of wrongdoing has been found and other countries in the European Union wanted the prosecutor fired as he was widely seen as failing to curb corruption.

House Democrats launched an impeachment inquiry against Trump this week over the affair.