Sen. Mitt Romney is pushing to subpoena testimony from former national security adviser John Bolton in the Senate trial over President Trump's impeachment.

The Utah Republican shared his thoughts with reporters on Capitol Hill on Monday just hours after Bolton said he would be willing to testify if he is subpoenaed, according to the Hill.

"I would like to be able to hear from John Bolton. What the process is to make that happen, I don’t have an answer for you," Romney, 72, said.

Fifty-one senators must vote to subpoena Bolton, meaning every Democratic senator and four Republicans must side together to reach the threshold.

Bolton, 71, resigned his position in the White House in September before the July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky became public knowledge. A whistleblower complaint about the phone call precipitated House Democrats' investigation and eventual impeachment of Trump.

Bolton issued a statement from his PAC saying he would not fight a subpoena to testify in the Senate trial, a shift in his stance from during impeachment proceedings in the House. He is one of four witnesses Senate Democrats have requested.

House Democrats considered subpoenaing Bolton to testify in their proceedings but decided against it knowing that the White House would claim executive privilege. Bolton had said that he would take any House subpoena to court and let a judge decide whether he should honor it or the White House's direction. The question was never settled during the House investigation.