President Donald Trump told reporters on Wednesday that he was opposed to his former national security adviser John Bolton testifying in his impeachment trial because "he knows some of my thoughts."

"He knows what I think about leaders," Trump said. "What happens if he reveals how I feel about another leader and it's not positive ... it would make the job a lot harder."

Bolton is a firsthand witness to the president's efforts to strong-arm Ukraine into pursuing investigations that would be politically beneficial to him while he withheld military aid and a White House meeting for Ukraine's president.

If the Senate allows Bolton to testify, he would be the highest-profile witness against the president to date.

Visit Business Insider's homepage for more stories.

President Donald Trump said on Wednesday that he was opposed to his former national security adviser John Bolton testifying in his impeachment trial because "it's a national security problem."

"He knows some of my thoughts," Trump told reporters. "He knows what I think about leaders. What happens if he reveals how I feel about another leader and it's not positive ... it would make the job a lot harder."

Trump's statement, however, underscores why Democrats believe Bolton's testimony is crucial: He "knows some of" Trump's thoughts.

The president made the comment hours before opening arguments are set to kick off in his Senate trial — only the third in US history. Trump was impeached last month on charges of abuse of power and obstruction of Congress.

Bolton, who announced this month that he was willing to testify in the impeachment trial, is a firsthand witness to the president's efforts to strong-arm Ukraine into pursuing investigations that would be politically beneficial to him while he withheld vital military aid and a White House meeting that Ukrainian President Volodymyr Zelensky desperately sought.

Bolton left his role in the Trump administration last summer but was privy to several key events at the center of the impeachment inquiry, according to his lawyer and public testimony from other current and former government officials.

The White House last year directed all executive-branch officials not to comply with congressional subpoenas for testimony. While more than a dozen lower-level officials refused to follow the White House's directive, all senior officials — including Bolton, Secretary of State Mike Pompeo, and Mick Mulvaney, the acting White House chief of staff — followed the order.

Bolton, if he testifies, could be the president's worst nightmare. As the former national security adviser, Bolton would be the highest-profile witness to testify against Trump and one who held frequent meetings with him.