So far, ten Republican senators have said that they are going to vote to disapprove the declaration — nearly as many as the number of House Republicans (13) who voted that way, even though there are 3 1/2 times as many Republicans in the House as in the Senate.
What accounts for the difference? A few possibilities, none exclusive of the others:
1) The House Republicans tend to represent voters who are more supportive of Trump than the Senate Republicans do. Take a House Republican and a Senate Republican from the same state, and the former is extremely likely to be from a part of the state that is more conservative than average.
2) Add in the fact that most senators aren’t up for re-election in 2020. The senators thus generally have less reason to worry about a primary challenge for a vote of disapproval, and more reason to worry about the general-election consequences of a vote of approval.
3) Senators tend to think more about the prerogatives of the institution than House Republicans do. It is tied up with their sense of self-importance, for good and ill.
4) The senators know that President Trump will veto the resolution of disapproval and that more than enough House Republicans will vote to sustain that veto. The senators may figure that they will get less blowback from strong supporters of Trump since the resolution is not going to stop him from getting his way.
Update: After looking at the final vote, in which 12 Republicans voted to disapprove the declaration, I think we can discount 2 as an explanation. The Republican senators at most danger of losing the general election — Martha McSally (Ariz.), Cory Gardner (Colo.), and Thom Tillis (N.C.) — voted with Trump.