Congressman Adam Schiff said he wanted Bolton to testify in the Trump impeachment investigation because he had mentioned the US pressure on Ukraine.

"John Bolton is a very important witness. According to some testimonies, Bolton was the one who expressed concern that the State Department was arranging an 'illegal transaction', implying a corrupt agreement. Bolton obviously has very relevant information and we want him to testify, "US House Intelligence Committee Chairman Adam Schiff said October 27 on ABC's" This Week ".

The US House of Representatives is opening an impeachment investigation against Trump on charges that the US President pressured Ukraine to investigate former Vice President Joe Biden. Witnesses interviewed in secret led Republican members to criticize the process as unfair.

Senator Schiff said the committees of the US House of Representatives will hold public hearings soon. "I don't want to give the exact time because we are struggling with our efforts to obstruct the investigation and prevent White House witnesses. I think they will resist our summoning of John Bolton," Schiff said.

An anonymous whistleblower working in US intelligence filed in August, said "had seen or heard the things that raised an urgent concern" and mentioned Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky. Trump is reportedly pressuring Zelenskiy to investigate Hunter Biden, the son of former vice president Joe Biden, who once worked at Ukrainian energy company Burisma.

On September 22, Trump confirmed he had discussed Biden's father and son with Zelenskiy and accused the two of corruption but gave no evidence. Trump's allegations are suspected of being politically motivated. Former Vice President Biden is likely to become a Democratic candidate in the 2020 US presidential election.

The US House of Representatives opened an impeachment investigation of Trump after a caucus meeting with Democratic congressmen on September 24. House Speaker Nancy Pelosi said Trump was responsible for actions that could have undermined national security and violated the US constitution. Trump immediately countered on Twitter and called the House of Representatives investigation a "trash witch hunt".

John Bolton became Trump's national security adviser in April 2018, but he resigned on September 10. After resigning, Bolton criticized Trump's policy on Iran, North Korea and Afghanistan. Trump said Bolton repeatedly hindered him in foreign affairs as a national security adviser.