Isaac makes a good point in response to my item about Obama trying to fight to a delegate draw in Pennsylvania. I agree that there have been times in the campaign when team Obama has seemed a little too obsessed with delegates, and not concerned enough with symbolic victories. (One example: The time Obama spent just prior to March 4 traveling to Rhode Island, where he wanted to limit Hillary's delegate advantage, might have been better spent in Texas, where he actually had a chance of winning--and, therefore, ending the entire race. In retrospect, he probably didn't get close enough in Texas for it to have mattered. But I'm not sure this was obvious ex ante.)
I think the reason for this obsession is that they really, really wanted to establish pledged delegates as the metric by which a nominee would be crowned. (I touch on this in my profile of Obama campaign manager David Plouffe this issue.) And if you start jumping around from metric to metric depending on which one favors you in a given moment, you quickly lose credibility with the media and superdelegates, as the Clinton campaign has discovered. On top of which, it's not entirely clear how big the tradeoff is between pursuing delegates and popular votes, at least within a given state. I'm sure it's not zero. But I'm not sure focusing purely on the popular vote as opposed to some combination of the popular vote and delegates in Pennsylvania would have netted Obama so many more votes overall. So I think the campaign's hyper-focus on delegates is pretty defensible.
Having said that, this does look like a moment when, if you have a chance to really hold down Hillary's popular vote total, you probably want to do it. Not just because this is really her last best hope for making up popular votes. But because, if you're able to hold her margin under five points in Pennsylvania, the ensuing media coverage could effectively force her from the race by drying up her fundraising. After all, as Chris notes, she's basically out of money already. Absent some really good news tomorrow, it's hard to see that changing.