For anyone who is still uncertain about whether it is electorally smart for the Democrats to listen to Mitch McConnell's advice or not (hey, he's more likely to be sincere than Pat Caddell!), Norman Ornstein and Tom Mann have a good column in The New Republic (via Yglesias) laying out the basic case. Jonathan Chait, who has been making that case throughout, chimes in again as well.
Just as a pure question of electoral politics, and just thinking about short-term questions (that is, the 2010 cycle), this isn't even a close case for the Democrats collectively, and that's the level at which Mann and Ornstein are looking at it. Of course, at an individual level it might be different, but for the most part, I don't think so. I'll go through this quickly...Democrats, collectively, are helped if Barack Obama's approval ratings are higher, and they are helped if rank-and-file Democrats are relatively more enthusiastic about the upcoming elections. On the other hand, it is possible that in at least some districts the vote itself, and the bill itself, will be unpopular.
For those who voted yes the first time around, I think the answer is clear: flip-flops are bad news, so in almost every case they are going to be better off sticking with the yes vote. The only exceptions are the Stupak group; they can plausibly argue that they have not flip-flopped at all, and on top of that they will take a hit to their pro-life support if they vote for the bill with the Nelson language instead of the Stupak language (never mind that there's apparently little or no actual difference; what matters is what the people who put together legislative scorecards will do).
For those who voted no the first time around, the calculus is different. If they believe that health care reform will be unpopular in November in their districts -- and there's certainly no guarantee of that -- then voting yes would expose them to some danger. At the same time, the defeat of the bill would also expose them to danger. So, once again, we're left with the same thing; there are quite a few Democrats in the House who want the bill to pass, but preferably without their vote. And while the group that is better off voting for a bill to pass is larger, it isn't large enough. Which basically gets everyone to where we are now, with the Speaker and the president working to coordinate things so that enough of those whose first preference is "bill passes, vote no" to accept their second choice, "bill passes, vote yes."