House Intelligence Committee Chairman Adam Schiff, D-Calif., said President Trump’s former National Security Adviser John Bolton is a “very important” witness in the House’s ongoing impeachment inquiry into the president.

Speaking on ABC News’ “This Week,” Schiff said Bolton has emerged as a key witness after hearing closed-door testimonies from other administration and government officials.

“Obviously he has very relevant information and we do want him to come in and testify,” Schiff said.

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The Democrat, however, noted that the White House probably will put up obstacles to getting Bolton in front of the committee.

“My guess is, they're going to fight us having John Bolton in,” Schiff added.

Closed-door interviews tentatively have been scheduled with Charles Kupperman, a Bolton deputy, and Tim Morrison, National Security Council's senior director for Russia and Europe. Kupperman and Bolton have both left the White House.

The Democrats have been investigating Trump's requests that Ukraine conduct certain investigations and whether the requests were in exchange for military aid. Trump requested the probes on a July phone call with Ukrainian President Volodymyr Zelensky.

Kupperman's interview is scheduled for Monday; Morrison's is set for Thursday. If Morrison appears for the interview, he will be the first White House aide to testify even as Trump has said his administration would not cooperate.

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Trump and other White House officials have insisted the president did nothing wrong.

The Associated Press contributed to this report.