Keep in mind, there's reports that McConnell wants Trump to be convicted, he just can't corral enough of his caucus so he's voting with the bulk of them. (So far, that's two votes on the trial's constitutionality--the second one declared it constitutional by a 56-44 vote, with Cassidy, Collins, Murkowski, Romney, Sasse and Toomey voting with the Democratic and Independent Senators; Cassidy voted to declare it unconstitutional the first time it came up but switched for this one.)
As a conservative acquaintance of mine noted, conservative legal opinion is pretty united on the question of whether the Senate can try Trump (they all say that it can), and so this is just a way for Republican Senators to signal on the one hand that they're pro-Trump while on the other hand giving themselves a fig leaf of cover to say that they think they shouldn't be voting on it in the first place, and so they'll vote to acquit on principle since a conviction would be an unconstitutional exercise of power.