Since the McCain-Feingold campaign finance reform legislation is clearly

benefiting Republicans and harming Democrats, why did most Republicans

vote against it and most Democrats support it?

There are cynical explanations. For example, maybe neither party predicted

the effects correctly. Maybe they all assumed that campaign

finance reform would have to be good for Democrats, and they voted pro

and con accordingly. Or perhaps the reform was viewed as bad for incumbents

as a group (which it is). Republicans may care more about protecting

incumbents, since they have majorities in both houses.

However, I think that a non-cynical explanation is at least partly

true. Republicans stood to gain from McCain-Feingold, but most were

still against it, because philosophically they oppose state regulation

of a financial exchange that they consider completely legitimate. Democrats

stood to lose from McCain-Feingold, but most voted for it, because philosophically

they oppose private financing of campaigns and they want to regulate

donations. Sometimes, arguments and reasons count.