(CNN) The team planning for President Donald Trump's impeachment trial in the Senate believes there are "significant and important" executive privilege issues regarding the potential testimony of former national security adviser John Bolton, according to a source familiar with the team's discussions.

The source told CNN on Friday that the Senate trial team sees Bolton's possible testimony as going "to the heart" of the President's constitutional powers under Article II.

Bolton shook the capital earlier this week when he said he is prepared to testify at Trump's impeachment trial, should he be subpoenaed to do so. Bolton issued a statement Monday after the courts did not rule on whether he would be compelled to testify during the House's impeachment proceedings, saying he was trying to meet his "obligations both as a citizen and as former national security adviser."

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

The former national security adviser would be a critical witness due to his firsthand knowledge of many of the events that led to Trump being impeached over his dealings with Ukraine. The President was impeached in December on two articles: abuse of power, for pressuring Ukraine to announce an investigation into Joe Biden's son, Hunter Biden, and obstruction of Congress, for ordering multiple top officials in his administration to ignore subpoenas for their testimony in the probe and ignoring subpoenas for relevant documents.

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