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 have already testified that he was deeply concerned about efforts by Trump and his allies to push Ukraine to open investigations into the president’s political rivals 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 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.