John Bolton, whom President Trump in September fired from his post as national security adviser, is reportedly willing to testify in the House impeachment inquiry, defying an order from the White House.

Bolton would testify “about his alarm at the Ukraine pressure campaign if a federal court clears the way, according to people familiar with his views,” The Washington Post reported Thursday. He is “expected to confirm their statements and describe his conversations with Trump, according to the people, who spoke on the condition of anonymity because of the ongoing inquiry.”

However, Bolton, a longtime GOP foreign policy adviser, does not want to comply with the Democratic inquiry without a court ruling on the ongoing constitutional dispute between the Trump administration and Congress, the people said. It remains unclear how quickly that could happen — and whether it would be in time for Bolton to be called as a witness in the public House impeachment hearings, which are scheduled to begin next week. On Wednesday, House Democrats said they are awaiting a key test case involving former White House counsel Donald McGahn, in which a district-court decision could come by the end of this month.

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His testimony, the Post wrote, “is expected to be ‘damaging’ to Trump, according to a person familiar with the matter.”

Weeks before Trump’s July 25 phone call with Ukrainian President Volodymyr Zelensky, the president put on hold $391 million in aid to the nation. He says he did so to pressure other nations to contribute more money and to take time to examine programs. Democrats, though, say it was a clear quid pro quo: Dig up some dirt on Joe Biden and his son, or no money.

In the call, Trump urged Zelensky to cooperate with his personal lawyer Rudy Giuliani on an investigation into Biden’s son, Hunter, who worked with a natural gas company owned by a Ukrainian oligarch. Joe Biden in 2016 reportedly called for the dismissal of a Ukrainian prosecutor who had reportedly been investigating allegations about Hunter.

Here’s exactly what Trump said regarding Biden’s son, according to a transcript released by the White House:

“I heard you had a prosecutor who was very good and he was shut down and that’s really unfair. A lot of people are talking about that, the way they shut your very good prosecutor down and you had some very bad people involved. Mr. Giuliani is a highly respected man. He was the mayor of New York City, a great mayor, and I would like him to call you. I will ask him to call you along with the Attorney General. Rudy very much knows what’s happening and he is a very capable guy. If you could speak to him that would be great. The former ambassador from the United States, the woman, was bad news so I just want to let you know that. The other thing, There’s a lot of talk about Biden’s son, that Biden stopped the prosecution and a lot of people want to find out about that so whatever you can do with the Attorney General would be great. Biden went around bragging that he stopped the prosecution so if you can look into it … It sounds horrible to me.”

As for a quid pro quo, here’s what Trump said:

“I would like you to do us a favor though because our country has been through a lot and Ukraine knows a lot about it. I would like you to find out what happened with this whole situation with Ukraine, they say Crowdstrike … I guess you have one of your wealthy people … The server, they say Ukraine has it. There are a lot of things that went on, the whole situation. I think you’re surrounding yourself with some of the same people. I would like to have the Attorney General call you or your people and I would like to get to the bottom of it. As you saw yesterday, that whole nonsense ended with a very poor performance by a man named Robert Mueller, an incompetent performance, but they say a lot of it started with Ukraine. Whatever you can do, it’s very important that you do it if that’s possible.”

The courts are now determining whether White House officials must testify if subpoenaed. Several cases are moving through the process. And The Post says “there is a major obstacle facing Democrats hoping to secure Bolton as a star witness: the court battle over congressional subpoenas will probably go to the Supreme Court and spill into next year.”