Decides to testify if subpoenaed

The formal national security advisor John Bolton has tweeted that he would testify in impeachment trial if subpoenaed.

Bolton criticized other Trump officials, including acting chief of staff Mick Mulvaney and US ambassador to the EU Gordon Sondland, for their involvement in efforts to pressure Ukraine to investigate Joe Biden.

In his statement, Bolton noted that he previously joined a lawsuit seeking a judge's ruling on whether to comply with a congressional subpoena or the White House's guidance not to cooperate with the House impeachment inquiry in the House. That decision was never decided but the House impeached the President without his testimony.

In order to testify in a trial in the Senate, there would need to be 4 Republicans to vote to subpoena Bolton to testimony. That is not a guarantee in the politically charged impeachment proceedings.