Former White House national security adviser John Bolton reportedly pushed President Trump to release military aid to Ukraine that the administration withheld until earlier this month.

At least a week before Trump is said to have pressured Ukrainian President Volodymyr Zelensky in a July 25 phone call to work with his personal attorney Rudy Giuliani to investigate Joe Biden's son, Hunter Biden, who did business in Ukraine while his father was vice president, the president ordered his acting chief of staff Mick Mulvaney to place a hold on the $400 million in funds, according to the Washington Post.

Trump says he was concerned about "corruption" in Ukraine and a senior administration official denied there was any "quid pro quo" to withhold the money unless Ukraine agreed to investigate Trump's potential rival in the 2020 election.

But other officials in Trump's administration were alarmed, partly because they were worried the block might be illegal. Bolton advocated for the release of the money to help Ukraine counter Russian-backed separatists in the eastern part of the country and a meeting between Trump and Zelensky.

Bolton, a former U.S. ambassador to the United Nations, was pushed out of the White House earlier this month, and has since slammed Trump's foreign policy on a number of fronts. He is also shopping around for a book deal and is said to have "a lot to dish."

Republicans on the Senate Appropriations Committee said the aid was withheld as the Trump administration determined whether Zelensky, an actor who took office in the spring, was pro-West or pro-Russia, and released the money when the Democrats threatened to block billions of dollars in Pentagon funding.

Democrats, concerned that Trump might be pressuring foreign countries to boost himself politically, are demanding answers. The controversy has also reenergized a push in the Democratic Party for impeachment. At the center of the standoff is an intelligence community whistleblower complaint that acting Director of National Intelligence Joseph Maguire refuses to give to Congress after receiving guidance from the Justice Department and White House. He is expected to testify to the intelligence committees this week.

Trump has admitted that he talked about Biden to Zelensky, but argues he did nothing wrong. Ukraine's foreign minister also claimed Trump did not pressure Zelensky during that phone call.

Some Republicans, including Trump himself, have called for the release of the transcript of his phone call with Zelensky. Reports have said the whistleblower complaint, which the intelligence community inspector general found to be "credible" and "urgent," relates to Trump's call with Zelensky, though House Intelligence Committee ranking member Devin Nunes warned there is no confirmation yet that it actually has anything to do with Ukraine.