A former member of the Ukrainian parliament and adviser to Ukraine's president told CBS News it was a "well-known fact" there that Donald Trump wanted "compromising" information on former Vice President Joe Biden. Serhiy Leshchenko added that Ukraine's president knew that U.S. aid to his country was at stake.

"I am sure that issue of Biden was forever on the table between Zelensky and Trump," said Leshchenko. As a former lawmaker and adviser to Ukraine's President Volodymyr Zelensky, Leschenko believes it was clear that President Trump wanted Ukraine to investigate his Democratic rivals.

Former member of the Ukrainian parliament and adviser to Ukraine's president Serhiy Leshchenko. CBS News

"Of course, he wanted political privileges, favors, for his re-election from Ukraine," he said.

"In return for military aid?" asked correspondent Roxana Saberi.

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"I would say yes," Leshchenko replied.

"Do you have any evidence of that?" asked Saberi.

"It was, like, well-known fact in Ukraine," Leshchenko replied.

In 2016, Leshchenko was at the center of exposing Paul Manafort's dealings in Ukraine. He said he recused himself from working with Zelensky in May this year, after it became clear that could threaten relations with President Trump's administration.

Ukraine relies heavily on U.S. aid in its war against Russia. But in July, Mr. Trump ordered nearly $400 million of that support withheld. Days later, in a phone call, he asked Ukraine's president to investigate the Bidens.

According to the whistleblower's complaint, Mr. Trump's personal lawyer, Rudy Giuliani, made or attempted contact with at least seven Ukrainian officials, including then-prosecutor general Yuri Lutsenko.

Lutsenko told the BBC that Giuliani asked him to investigate the Bidens.

BBC correspondent Jonah Fisher asked Lutsenko, "Have you got any evidence that Joe Biden acted in any way which supported Hunter Biden's company, Burisma?"

"It is not my jurisdiction," he replied.

"Under Ukrainian law, you've got nothing?"

"Nothing, certainly," Lutsenko said.

But Serhiy Leshchenko says the two men were circumventing official channels.

He also told Saberi that Giuliani wanted to meet President-elect Zelensky before his inauguration in April, but that Zelensky said no, because he realized that "everything behind the story is toxic."

President Zelensky said Monday he would not release his transcript of his phone call with President Trump.