As you can see by my nickname, I vote straight Democratic. I believe those who are serious about helping the Democratic Party, should do this, with rare exception.
I constantly hear people say that they vote for the person, not the party. Of course, I hear it less here, but I still hear it. I wonder where people got this idea? I think maybe our teachers put this in our heads in school. Anyway, it sounds very enlightened and independent. Years ago, when there were liberals in the Republican Party, there was more going for it. However, I think that in almost all cases splitting your ticket is a bad idea.
The main reason, as we just saw, is that when we vote for Congress, we don't vote for just the candidate, but we vote for the Congressional leadership as well. To paraphrase a recent letter from Michael Moore, 'all that matters in the number of Ds and number of Rs, and that's true even if the R takes your position and the D takes the wrong position.' This extends down to state legislatures, who are responsible for redistricting. If you vote D for Congress and R for state legislature, the state legislature may just redistrict your D congressperson out of existence. Again with the Governor. when a Senator dies or retires the Governor appoints the new Senator. So if you split you ballot between Senator and Governor, you are diluting your vote for Senator.
Another reason to always vote Democratic is that a Democrat brings a Democrat staff and newtwork of connections along to the job. A Reput brings a reput crew.
The only reason I can see for voting for a Republican is if the Democratic is totally unacceptable. I'll still vote for him if he's lousy, crummy, ordinary, piss poor, jerk or ethically challenged. I'm won't vote for a KKKer, right wing troll who got on the ballot while the Party was asleep, or a convicted murderer.
The only time I ever voted for a Republican was when a winger slipped on to the ballot while the Dems were napping in Delaware.
UPDATE: Another reason for party line partisan occured to me, at least at the polling place. The Republicans have been very partisan and disciplined, and it helps us to have a united front.