Some diarists have noted that it would be hard for red state democratic senators to vote against Alito. They wouldn't get reelected and then we definitely wouldn't have Harry Reid as majority leader. So it's important they get reelected. That's the claim.
But the job of a senator really is not to get reelected. It's to vote on and legislation and nominations and things like that. And it isn't even to do the will of the people who elected them because they don't know what that is. They have the whole election season to convince their constituents that they actually did the right thing.
So if a Supreme Court nomination is one of the most crucial events these senators will vote on (and the Alito one in particular), then this is one vote on which senators should vote what they think and set for themselves the job of convincing their constituents. Not having Alito on is more important than personal political calculus. In this case, being a red state Democrat is not a reason, it's an excuse.