CNN:

Former White House national security adviser John Bolton said Monday he is willing to testify in the Senate's impeachment trial of President Donald Trump if Bolton is subpoenaed.

"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," Bolton said in a statement. "I have concluded that, if the Senate issues a subpoena for my testimony, I am prepared to testify."

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