The impeachment enquiry into US President Donald Trump formally moved into its public phase, and witnesses in the inquiry are beginning to testify in an open session before Congress.

Fiona Hill, former National Security Council senior director for Europe and Russia, told the House Intelligence Committee on Thursday that she was told Trump’s personal lawyer, Rudy Giuliani, was doing “explosive” things that would “probably come back to haunt us.”

Hill said her former boss John Bolton called Giuliani “a hand grenade,” adding that the former national security advisor was then referring to the lawyer’s TV appearances floating what Democrats called conspiracy theories about the 2016 US presidential election, former US Vice President Joe Biden, and his son Hunter Biden.

On Thursday, David Holmes, a staffer from the US Embassy in Ukraine, who also testified described a conversation he overheard between Trump and US Ambassador to the European Union Gordon Sondland, which occurred on July 26, a day after a Trump-Zelensky phone call that triggered an anonymous whistleblower complaint and the impeachment inquiry.

However, few are expecting dramatic new revelations, since these witnesses have testified behind closed doors already. The hope, for Democrats, is to throw more of a public spotlight on what Taylor and Kent are saying — and, in contrast, Republicans are hoping to discredit them.

Last week, President Trump described the impeachment probe against him as “witch hunt”, saying he was “too busy” to watch it.

In October, President Trump opposed impeachment enquiry, saying that there should be no public hearings during the House of Representatives’ impeachment enquiry against him, and directed White House Chief of Staff Mick Mulvaney not to appear before the committees investigating Trump’s phone call to Ukraine.

After almost a month of calling for greater transparency in the enquiry, the White House changed its strategy this week by prohibiting several of its officials from even testifying behind closed doors before the lower house committees.

Late September, the impeachment inquiry, which House Speaker Nancy Pelosi initiated over a complaint by an anonymous whistleblower, is looking into White House’s alleged efforts to withhold military aid to have Ukraine investigate a Trump’s political rival, Joe Biden.