"Ex-president has been watching his trial from Mar-a-Lago estate in Florida"
"Multiple people tell me Trump was basically screaming as Castor made a meandering opening argument that struggled to get at the heart of the defence team's argument," said CNN's chief White House correspondent Kaitlan Collins.
The trial moved forward on Tuesday after six Republicans joined Democrats in a 56 to 44 vote that ruled Trump's second trial is constitutional.
Earlier Mr Miller told Fox News, that Mr Trump was in a "great mood."
"The president was in a great mood. First of all, he's in Palm Beach ... But otherwise it was a quick conversation, because I had to join Fox News,” he said.
A total of 67 senators will be needed to vote in favour of convicting Mr Trump at the end of his trial.
If that supermajority mark is passed and the ex-president is convicted, the Senate can take action against him and vote by a simple majority to disqualify Mr Trump from ever holding elected office again.