I think that most Congressmen, overwhelmingly of the professional class, like other members of the professional class are smarter than average, maybe standard definition plus or minus. Say, IQs of 100 to 140. I think that really intelligent Congressmen are quite rare.
I think that pertains to the Congressional caucuses of both parties. Republicans aren’t particularly stupid. Democrats aren’t particularly smart. Or vice versa. I’d bet a shiny new dime that 90% of Congressmen fall within the range I’ve suggested and that there’s very little difference between the Republican Congressional caucus and the Democratic one. If anyone has hard evidence (i.e. not based in what they say or what they support or a priori considerations), I’d appreciate it.
You can be smart and wrong. You can be dumb and right.
I think that resorting to notions like intelligence to explain differences of opinion is an error. I also think that attributing differences of opinion to malice is usually an error. Differences in preference are enough to explain many differences of opinion.