This will be cast as an enormously risky decision, because if Coakley loses Obama loses “prestige” or whatever. Of course, Obama stands to lose a lot more than just prestige in the event of a Coakley loss, so why not pull out all the stops?I agree with the first point. To the extent that a presidential appearance matters, Obama is correct to use that resource here. Not only is the seat more important than his prestige, but to the extent that prestige is important, he's going to suffer if Coakley loses regardless of his role in the campaign.
Either way, it’s a possible sign that internal tracking polls are not looking good for Coakley right now.
But the second point doesn't make sense. I'll run the possibilities. The givens here are that everyone at this point believes it's a competitive contest, and that for Obama the direct costs of a trip up to Boston (a little money for the expedition, a little time lost on a weekend) are very small.
1. Internal tracking shows Coakley is losing badly. In that case, an Obama appearance will make no difference, and all that is on the line is prestige, with the question being which is worse: campaigning and losing, or not campaigning and losing? Not an obvious call, it seems to me, leaving two plausible choices.
2. Internal tracking shows a very close race. Now, the loss of the seat is what really mattes, and Obama should certainly go.
3. Internal tracking shows that Coakley will win. Once again, only prestige is on the line, but now it's pretty clear that it's better for Obama to go (and get credit for the win) rather than stay home and let others take the credit.
So, Obama showing up doesn't actually tell us anything; his incentive is to act that way if she's winning, if it's close, and probably if she's losing. Not going would have been bad news for Coakley, because that would only be a plausibly smart move for Obama if he believed she was probably going to lose -- but again, it's also plausibly the right move to go even under that condition.
The calculations are different for the November campaign, because in that case attending one rally means skipping another one. But here, he's only missing however long it takes him to actually attend the event, which I think we can assume is negligible.
By the way, my guess is that Coakley will survive -- I agree with those who think that Brown probably peaked too soon. Higher turnout should help the Dems in MA, and anything that throws attention on the contest should increase turnout (which is why Obama's visit may actually be more helpful than a typical trip of this kind). Had Brown's surge happened a little later, and therefore generated less media attention before the vote, it probably would have helped him. I should say, however, that this is basically just a guess, not a hard prediction.