Trump Asks China For Help Over Biden

US President Donald Trump openly called on China as well as Ukraine to investigate his potential 2020 election rival Joe Biden, doubling down as Democrats began interviewing witnesses in an impeachment investigation that has rocked the White House.

Facing possible removal from office precisely for seeking foreign help against his political challengers, Trump said he wanted Ukraine President Volodymyr Zelensky and Chinese leader Xi Jinping to go after Biden, the leader in the race for the Democratic presidential nomination next year.

"I would say that President Zelensky, if it were me, I would recommend that they start an investigation into the Bidens," Trump told reporters outside the White House.

"If they were honest about it they would start a very simple investigation on the Bidens," Trump said of Ukraine.

"Likewise, China should start an investigation into the Bidens, because what happened in China is just about as bad as what happened with Ukraine."

Asked if he would ask Xi to do the same, Trump replied, "It's certainly something we can start thinking about."

In a statement, Biden's campaign called the president's comments "a grotesque choice of lies over truth and self over the country."

Kate Bedingfield, Biden's deputy campaign manager, said Trump was "desperately clutching for conspiracy theories that have been debunked and dismissed by independent, credible news organisations."

"Donald Trump is terrified that Joe Biden will beat him like a drum," she added.

Trump ramped up his effort to shape the public narrative just as the first major witness in the impeachment probe arrived in Congress to provide testimony on allegations that the White House illicitly sought political help from Ukraine for the 2020 election.

Former US special envoy to Ukraine Kurt Volker was caught up earlier this year in the efforts by Trump and Trump's lawyer Rudy Giuliani to pressure Ukraine to investigate Biden and Biden's son Hunter, who had business ties to a Ukraine gas tycoon.

Those efforts culminated in a July 25 phone call between Trump and Ukraine President Volodymyr Zelensky, in which Trump appeared to tie getting dirt on Biden to military aid for Ukraine.

A US intelligence officer's whistleblower complaint about the impropriety of Trump's actions, and a White House record of the call, both released last week, drove Democrats to open the impeachment investigation, alleging that the president betrayed his oath of office and jeopardised national security.

Volker could strengthen the Democrats' case. The whistleblower complaint depicts him speaking with Giuliani "in an attempt to 'contain the damage' to US national security" of Giuliani's meddling in Ukraine affairs.

A day after the July 25 call, Volker met with Zelensky and other senior Ukraine officials and advised them "about how to 'navigate' the demands that the president had made of Mr. Zelensky."

House investigators received more potential evidence against Trump Wednesday when the inspector general of the State Department handed over a package of materials sent to Secretary of State Mike Pompeo earlier this year to support an investigation of the Bidens.

Giuliani told US media that he sent the package, which was in an envelope labelled "White House."

The Giuliani package contained "disinformation, debunked conspiracy theories, and baseless allegations" regarding the Bidens, Democrats said in a statement after meeting the inspector general.

"These documents also reinforce concern that the president and his allies sought to use the machinery of the State Department to further the president's personal political interests," they said.

Trump, meanwhile, pressed forward with his strategy of deflecting the allegations by focusing the narrative on the Bidens, as his Republican allies in Congress sought to put brakes on the impeachment probe.

House Minority Leader called on House Speaker Nancy Pelosi to suspend the inquiry until "transparent and equitable rules and procedures" could be established.

Anything less than a "through, transparent, and fair process would represent a supreme insult to our Constitution," he said.

Trump and Giuliani allege that Biden, as vice president, tried to block a Ukraine corruption probe into his son's business partner, using US aid as leverage.

But the record shows that there was no investigation, and that Biden in fact viewed Kiev's corruption prosecutor at the time as himself deeply compromised.

Trump is also seeking to paint his Democrat accusers "liars."

"Schiff has now been proven to be a liar... he's a stone-cold liar," Trump told reporters.

"The whole investigation is crumbling."

Opinion polls showed increasing public support to impeach the president, but also suggested his strategy to hurt Biden was having an impact.

USA Today said 45 percent of Americans support an impeachment vote, against 38 percent opposed, according to a USA Today/Ipsos poll published Thursday.

The poll, however, also showed 41 percent in favour of examining the Bidens' ties to Ukraine while 21 percent were opposed. (AFP)