Ukrainian President Volodymyr Zelensky (L) was allegedly pressured by President Donald Trump's emissaries to investigate political rival Joe Biden. (AFP)

Democratic lawmakers on Friday demanded that the White House turn over documents related to allegations that President Donald Trump pressured Ukraine for political favors, as the explosive impeachment investigation against the US leader intensified.

The congressional committees leading the probe cranked up the heat on the White House as evidence mounted that Trump illicitly used his office to enlist Ukraine President Volodymyr Zelensky’s help to damage 2020 Democratic rival Joe Biden, in exchange for military aid.

“The White House has refused to engage with — or even respond to — multiple requests for documents,” the Democratic chairmen of the House oversight, intelligence and foreign affairs committees said.

“After nearly a month of stonewalling, it appears clear that the President has chosen the path of defiance, obstruction, and cover-up.

“His actions have left us with no choice but to issue this subpoena.”

In their letter to acting White House Chief of Staff Mick Mulvaney, the committees demanded that he turn over the requested files by October 18.

The subpoena followed a demand earlier Friday for documents from Vice President Mike Pence.

The investigators pointed to Pence’s knowledge of Trump’s calls to Zelensky and his own meeting on September 1 with the Ukraine leader, as well as discussions he may have had with Trump and US diplomats about Ukraine and obtaining political dirt on Biden.

A series of text messages between US diplomats dealing with Ukraine, made public by the congressional investigators, supported Democratic accusations that Trump had illegally sought foreign help for his reelection effort.

And the Wall Street Journal reported that, in an interview, Republican Senator Ron Johnson said he had learned that a quid pro quo had been proposed to Zelensky’s government by Trump’s emissaries, tying military aid to a Ukraine corruption investigation into Biden.

Trump pushed back hard, saying there was no quid pro quo and, in an effort to recast the entire saga, insisted it was his responsibility to investigate “corruption.”

“I don’t care about Biden’s campaign but I care about corruption,” he told reporters.

“I don’t care about politics,” he said. “I believe there was tremendous corruption with Biden.”

The former US vice president responded by calling Trump “the most corrupt president we’ve had in modern history.”

“I am not going to stand for it,” Biden said at a campaign event in Los Angeles, deeming Trump “unhinged.”

“He has indicted himself by his own statements,” Biden added, one day after Trump openly called for both Beijing and Kiev to investigate the Democrat for corruption.

Trump has alleged that Biden’s son Hunter earned “millions” from sitting on the board of directors of a Ukraine tycoon’s gas company.

But no evidence has surfaced showing wrongdoing on the part of either Biden.

The diplomats’ text messages showed that some had doubts about Trump’s pressure on Zelensky, including allegedly freezing a $400 million military aid package.