The Senate's impeachment trial is expected to formally begin when senators and Chief Justice John Roberts are sworn in on Thursday. Senators are expected to pass a resolution on the rules for the trial on Tuesday and start opening arguments later in the week.

The first phase of the trial — opening arguments and questions from senators — is expected to last roughly two weeks. That would put the end of that phase up against or potentially past the State of the Union.

After that senators are expected to have to decide on whether to call additional witnesses or request documents, a decision that is putting a spotlight on divisions within the Senate Republican caucus.

Democrats have requested four witnesses, including Bolton and acting chief of staff Mick Mulvaney . They need four GOP senators to give them the simple majority needed to call a witness.

Graham on Wednesday said he hopes that "nobody will be called as a witness" and warned Republican senators against believing that Democrats want to get to the bottom of Trump's decision to delay Ukraine aid.