The subject of the latest round of impeachment calls against President Donald Trump is a July phone call Trump made to newly-elected Ukranian President Volodymyr Zelensky, in which he reportedly threatened to withhold American aid if Zelensky did not investigate former Vice President Joe Biden and his son Hunter Biden for unsubstantiated charges of corruption.

On Tuesday, Trump said he would release the transcript of the conversation. John King, CNN’s chief national correspondent, said that while he has not seen the transcript of the conversation, sources close to the administration say they are concerned with its content.

“I’ve talked to a couple of Republicans who’ve talked to senior White House officials who say they’re a bit nervous about this,” King said during an interview with Boston Public Radio on Tuesday.

King said the revelations of Trump’s alleged attempt to smear Biden has emboldened House Democrats who were already in favor of impeachment, and may be enough to change the minds of Democrats who are hesitant to begin impeachment hearings.

“It is without a doubt that the strength ... among House Democrats is growing,” King said. “Having lived through the last impeachment, this is a big deal. We are at a very important time here.”