The Hotline's Reid Wilson had a good item yesterday contrasting Dick Lugar and Orrin Hatch and concluding that "campaigns matter."
I think that's right, but I'd urge a bit of caution. On the one hand, campaigns are apt to matter most in exactly the situation Wilson is focused on: primary elections. In general elections, especially in high-information general elections, campaigns are not going to be able to overwhelm party identification and whatever else is out there. Campaigns can still have an effect, even in presidential general elections, but there's just no way that they're going to do nearly as much.
And then the second thing is that a lot of the differences between what Lugar and Hatch have done go beyond what we usually think of as "campaigns." Both Senators made choices about what to spend their time on in Congress, and how to vote on potentially controversial items. Both of those (sets of) choices may have affected their re-election bids. We certainly can call all of that part of the campaign, and it's true that looked at from one angle it's reasonable to think of everything that politicians do as campaign-related...but looked at from another angle, there really is a separation of "governing" and "campaigning." My sense of these things is that neither of these points of view is right or wrong; they are both true, both for those of us who look at it from outside and for the politicians themselves.
So, yes, campaigns can and do matter, but once you get beyond that it starts getting very complicated very quickly.