Former national security adviser John Bolton said Monday that he would testify in President Trump's impeachment trial should the Senate issue a subpoena.

Why it matters, via Axios' Jonathan Swan: Bolton was the most prolific note-taker at the top level of the White House and probably has more details than any impeachment inquiry witness, so far, about President Trump's machinations on Ukraine.

"Bolton was a voracious note-taker, in every meeting," said a source who attended numerous meetings with him.

While others sat and listened in meetings with Trump, Bolton distinguished himself by filling legal pads with contemporaneous notes on what was said in the room.

Trump's former top Russia expert Fiona Hill testified in the House impeachment inquiry that Bolton had concerns about pressuring Ukraine to investigate the president's political opponents and that he told her to alert White House lawyers.

Our thought bubble, via Axios' Margaret Talev: This development suggests Bolton has something to say — and wants to say it.

But it's not necessarily clear that his testimony would help Democrats. Even if he disagreed with Trump's decision to withhold military aid from Ukraine, he may not believe it is impeachment-worthy

Regardless, this — plus the U.S. strike on Iranian Gen. Qasem Soleimani — has put Bolton, a noted Iran hawk, squarely back in the news and in the driver's seat, to some extent.

The intrigue: Senate Majority Leader Mitch McConnell (R-Ky.) dismissed Minority Leader Chuck Schumer's (D-N.Y.) call last month to have four White House witnesses, including Bolton, testify in the Senate's impeachment trial.

McConnell argued that it was the House's "duty to investigate" and that the Senate will not volunteer its time for a "fishing expedition."

Bolton expressing his willingness to testify will be sure to cause Senate Democrats to put even more pressure on McConnell — and, perhaps more importantly, a group of swing Senate Republicans — to consider allowing him to speak.

What to watch: A simple majority in the Senate — which currently consists of 53 Republicans and 47 Democrats or independents — is needed to call impeachment witnesses. Expect moderate Republicans like Sens. Mitt Romney (R-Utah), Susan Collins (R-Maine) and Lisa Murkowski (R-Alaska) to face pressure.

Schumer said in a statement Monday: “Given that Mr. Bolton’s lawyers have stated he has new relevant information to share, if any Senate Republican opposes issuing subpoenas to the four witnesses and documents we have requested they would make absolutely clear they are participating in a cover up.”

What they're saying:

"The House has concluded its Constitutional responsibility by adopting Articles of Impeachment related to the Ukraine matter. It now falls to the Senate to fulfill its Constitutional obligation to try impeachments, and it does not appear possible that a final judicial resolution of the still-unanswered Constitutional questions can be obtained before the Senate acts.

Accordingly, since my testimony is once again at issue, I have had to resolve the serious competing issues as best I could, based on careful consideration and study. I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify."

— John Bolton

A spokesperson for Bolton declined to comment when asked if he would comply with a House subpoena.

Go deeper: Trump aides fear John Bolton's secret notes