Former national security adviser John Bolton is willing to defy the White House and testify in the House impeachment inquiry about his alarm at the Ukraine pressure campaign if a federal court clears the way, according to people familiar with his views. 

Bolton could be a powerful witness for Democrats: Top State Department and national security officials already have testified that he was deeply concerned about efforts by Trump and his allies to push Ukraine to open investigations into a political rival of the president's while the Trump administration held up military aid to that country.

The former national security adviser, who abruptly left his post in September, is expected to confirm those witnesses' 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 impeachment hearings, which are scheduled to begin next week in the House. On Wednesday, House Democrats said they are awaiting a key test case involving former White House counsel Donald McGahn, in which a decision by a district court could come by the end of this month.