Paul's motion was killed on a 55-45 vote, with five Republicans joining all Democrats, meaning 45 Republicans voted for Paul's effort. Republican Sens. Mitt Romney of Utah, Ben Sasse of Nebraska, Susan Collins of Maine, Lisa Murkowski of Alaska and Pat Toomey of Pennsylvania crossed party lines to vote with Democrats.

In order to convict Trump at his trial, at least 17 Republicans will need to vote with all Democrats when the trial begins next month. Significantly, Senate Republican leader Mitch McConnell sided with Paul in the vote -- a potential indicator that he agrees the constitutionality of impeaching a former President is in question.

Paul argued after the vote that the fact that 45 Republicans sided with him "shows that impeachment is dead on arrival."
"If you voted that it was unconstitutional then how in the world would you ever hope to convict somebody for this?"

Paul asked, adding ,"45 of us, almost the entire caucus, 95% of the caucus, voted that the whole proceeding was unconstitutional. This is a big victory for us. Democrats can beat this partisan horse as long as they want -- this vote indicates it's over, the trial is all over."

Paul, a Kentucky Republican, told reporters earlier in the day, "I think there will be enough (votes) to show that more than a third of the Senate thinks that the whole proceeding is unconstitutional, which will show that ultimately they don't have the votes to do an impeachment."