Then-President-elect Donald Trump meets with former New York City Mayor Rudy Giuliani at the clubhouse of Trump National Golf Club November 20, 2016 in Bedminster, New Jersey.

Presidential lawyer Rudy Giuliani said Monday he can't be "100%" certain that Donald Trump did not threaten to withhold foreign aid when Trump reportedly urged Ukraine's president to investigate the son of 2020 Democratic frontrunner Joe Biden.

Giuliani said in a Fox Business interview Monday that it was a "false story" to suggest that Trump had used foreign aid as leverage in a July phone call with Ukraine President Volodymyr Zelensky.

But "I can't tell you if it's 100%" false, he added.

Trump's call with Zelensky is reportedly the subject of a whistleblower's complaint that has sparked a disclosure fight between government branches. It has also spurred new accusations of wrongdoing — and calls for impeachment — from Democrats.

The Wall Street Journal reported that Trump had asked Zelensky "about eight times" in the call to work with Giuliani to investigate the former vice president's son Hunter over his past role with a Ukraine-based natural gas company.

The former vice president said such an action by Trump would amount to an abuse of power.

"If these reports are true ... it means that he used the power and resources of the United States to pressure a sovereign nation — a partner that is still under direct assault from Russia — pushing Ukraine to subvert the rule of law in the express hope of extracting a political favor. Such clear-cut corruption damages and diminishes our institutions of government by making them tools of a personal political vendetta," Biden said in a statement Friday.

Giuliani claimed Monday that Hunter Biden received millions of dollars in laundered money, and has accused Joe Biden of political corruption for pressuring Ukraine to fire a prosecutor who reportedly oversaw a probe into the owner of that gas company, Burisma Holdings.

GIULIANI TWEET Today though it's the $3 million laundered payment, classical proof of guilty knowledge and intent, that was kept from you by Swamp Media. Ukraine-Latvia-Cyprus-US is a usual route for laundering money. Obama's US embassy told Cyprus bank not to disclose amount to Biden. Stinks!

There's no clear evidence that Biden's actions as vice president were intended to help his son. Hunter Biden has not been accused of illegal wrongdoing related to his work with the company, The New York Times reported.

A spokeswoman for Giuliani did not immediately respond to CNBC's request for further comment on his remarks.

The intelligence community's inspector general, Michael Atkinson, told lawmakers that the whistleblower's complaint was "urgent" and "credible." But Joseph Maguire, acting director of national intelligence who succeeded Dan Coats in July, claimed that the complaint does not qualify as an "urgent concern" under the law and therefore doesn't have to be shared with congressional intelligence committees.

House Intelligence Committee Chairman Adam Schiff, D-Calif., issued a subpoena to Maguire on Sept. 13. Maguire is expected to testify before that committee on Thursday in a public hearing.

Several Democrats have said that the reports suggest Trump attempted to solicit a foreign government to attack his political rival ahead of the 2020 election. Some have also questioned whether Trump used foreign aid to Ukraine as leverage in the conversation.

Sen. Chris Murphy, D-Conn., a Senate Foreign Relations Committee member, told MSNBC he had recently met with Zelensky, who "was very concerned why the security aid was being cut off."

"Even if there wasn't this explicit promise made that if you investigate Biden, we'll give you your aid, that certainly was the impression among many in Ukraine who couldn't help but connect these demands they were getting from the president's political representatives to investigate his opponents, and the sudden surprising cutoff of security aid," Murphy said in an MSNBC interview Friday.

The Trump administration withheld sending hundreds of millions of dollars in military aid for Ukraine until earlier in September, and did not provide a clear reason for the delay, The Washington Post reported.

On Sunday, Trump acknowledged bringing up Biden during the call with Zelensky but maintained that it was a "perfect conversation" and that there was "absolutely nothing wrong" with the discussion.

Giuliani defended Trump and his call in the Fox Business interview Monday morning.

"No money was mentioned, no quid pro quo," Giuliani said. He added that the call was "proper action by a president" — and while Democrats might disagree, it's "certainly not grounds for impeachment."