Ex-White House National Security Adviser John Bolton said on Monday that he was willing to testify in the impeachment trial of his former boss, Donald Trump, in what could present a challenge for the U.S. president. 

Testimony from Bolton, who has reportedly likened Trump’s dealings with Ukraine to a “drug deal”, could sway lawmakers and the public that the president had indeed abused his power to hurt a prominent Democratic political rival in this year’s presidential election.

In a blog post on his website on Monday, Bolton, also a former ambassador to the United Nations and hawkish television pundit, laid his reasoning in favor of giving testimony in what has proven to be a divisive impeachment process.

“During the present impeachment controversy, I have tried to meet my obligations both as a citizen and as former National Security Advisor,” wrote Bolton.

“I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify.”

Should he testify, Bolton would be the most senior former Trump adviser to give evidence in an impeachment crisis that, while front-page news, has never appeared to seriously threaten Trump, who maintains strong support from his fellow Republicans in Congress.

Bolton released his statement after a court decided not to rule on whether he was obliged to testify. The Republican-dominated Senate has yet to decide on what format Trump’s trial will take, following his impeachment late last year by the lower House of Representatives.

The Democratic-led House launched impeachment proceedings against Trump on Sept. 24 following claims by a whistle-blower that the commander-in-chief had sought to pressure Ukraine to interfere in the 2020 US presidential elections.

In a July 25 phone call, Trump allegedly made nearly $400 million military aid to help Ukraine fight Russian-backed separatists and a coveted White House meeting for Ukrainian President Volodymyr Zelensky contingent on a “quid pro quo” arrangement.

In return, Zelensky was supposed to open corruption probes into Joe Biden, a former U.S. Vice President, and his son, Hunter, who served on the board of Ukrainian gas company Burisma, as well as into alleged Ukrainian meddling in the 2016 U.S. election.

The elder Biden is a leading candidate in the race to win the Democratic nomination and challenge Trump in 2020. Trump, a Republican, has accused Democrats of time-wasting and says the inquiry amounts to a “witch hunt”.