The trial could take a volatile turn if senators decide to call witnesses such as John Bolton, his former national security adviser, who has offered to testify if subpoenaed. Trump has said he would invoke executive privilege, the legal principle that allows presidents to keep confidential his conversations with aides, if that happens.
It's unclear whether the Senate would respect that claim, and at least four Republicans would need to join with Democrats to call Bolton as a witness.
Another unpredictable factor is Rudolph W. Giuliani, the former New York mayor who also represents Trump as a personal lawyer. Giuliani has advocated far-fetched legal theories on Twitter, such as urging the Supreme Court to toss the case against the president.
In addition, Giuliani's role in the impeachment trial is more akin to a witness -- or a potential suspect -- than a lawyer.
He has served as Trump's point man on trying to dig up dirt in Ukraine on Joe Biden, the former vice president who is seeking the Democratic presidential nomination. Giuliani refused to respond to a subpoena from the House, and he's continued traveling to Ukraine to promote unfounded allegations about Biden and his son, Hunter, who served on the board of a Ukrainian gas company.
It's unclear how long Trump's trial will last. President Bill Clinton, who was impeached for lying to a grand jury about his affair with a White House intern, was acquitted by the Senate in 1999 after five weeks of proceedings.
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