Fox News legal analyst Andrew Napolitano Andrew Peter NapolitanoFox's Napolitano says grand jury erred in Taylor case: 'I would have indicted all three of them' Fox's Napolitano: Supreme Court confirmation hearings will be 'World War III of political battles' Fox's Napolitano: 2000 election will look like 'child's play' compared to 2020 legal battles MORE said Monday that the allegation that President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE pressured the leader of Ukraine to investigate the son of 2020 presidential candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE could be the most “serious charge he’s faced” since entering office.

“I think this is the most serious charge against the president, far more serious than what Bob Mueller dug or dragged up against him, if there was a quid pro quo,” Napolitano said on Fox Business Network, alluding to the report the former special counsel prepared on Russian election interference and accusations that Trump obstructed justice.

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The comments came as Trump faces increased scrutiny regarding reports that he pushed Ukrainian President Volodymyr Zelensky in a July phone call to investigate Biden's son, Hunter Biden, over allegations of corruption.

The call occurred around the same time the Trump administration decided to delay $250 million in military aid to Ukraine, raising speculation from some as to whether the talks were apart of a quid pro quo effort. Trump earlier this month released the aid after lawmakers raised concerns about the delay.

Trump on Monday denied making mention to military aid during his discussions with Zelensky, telling reporters at the United Nations General Assembly that he "did not make a statement that you have to do this or I’m not going to give you aid."

"I wouldn't do that. With that being said, what I want is I want — you know, we’re giving a lot of money away to Ukraine and other places. You want to see a country that's going to be not corrupt," Trump said.

Napolitano pushed back on Trump's argument, saying that "if you are the president of the United States and making a conversation that you know your intelligence community is listening to, of course you’re not going to articulate a quid pro quo."

"You’ll just make the quid pro quo happen," he said.

Napolitano later argued that the controversy was problematic for Biden's presidential prospects as well, saying that it "ought to be the end of his dream for the presidency." But he added that it didn't "diminish one iota what the current president is doing."

"If true, we haven’t seen the whistleblower complaint, and, under the law it has to be revealed — if true, this is an act of corruption," he said.

Trump has repeatedly moved to portray Biden as a corrupt politician over his efforts to dismiss a top prosecutor in Ukraine while he was vice president in 2016. The prosecutor was probing a natural gas company owned by a Ukrainian oligarch and which Hunter Biden sat on the board of at the time.

There is no evidence that suggests Biden was working in his son's interests during his efforts.