Working from Chris Cillizza's numbers, Obama came into tonight with a lead of 1579-1473, and those numbers include pledged delegates and committed superdelegates. He'll take the larger share of Mississippi as well, so that lead will widen slightly. His wins in TX, VT, WY and MS pretty much offset HRC's big day last week. And there's another factor to consider: in the week since the OH-TX-RI-VT primary, Obama's won the endorsement of 14 superdelegates to Clinton's six. (Plus she's going to lost Spitzer, and we can all but pencil Pelosi into the Obama column as well.) Should he continue his current pace of attracting SDs, there's a chance that he won't even be trailing in this category by the time Pennsylvania hits. From there, delegate apportionment alone should secure the nomination.
If Obama wants to net himself a potential game-changer in the meantime, he ought to respond to the challenge thrown down by those Ohio superdelegates who want a "concrete plan" for job retention from each candidate before they commit an endorsement. I have to say, I admire the proactive stance of these superdelegates: when you consider that the reality is that most of the SDs are likely waiting out the process to see what sort of political cover the primaries will afford them, the fact that there are some SDs who are willing to stake some political capital in getting a good deal on paper for their constituents is laudable.
UPDATE: As Matty points out, as the numerator diminishes, so does the denominator.
On March 1, everyone thought Clinton was going to lose this race for the nomination. With no further contests left in March there's been no net change in pledged delegates but there are 400+ fewer pledged delegates still at stake. Clinton was drawing dead on March 1, and she's drawing dead on March 12. Even the 12 point win Clinton's probably looking at in Pennsylvania can't genuinely turn this around for her.The opportunities to close the gap disappear faster than the votes that actually close it. Clinton could win the PA delegates by a 36 point margin, but David Axelrod will whistle and skip and say goodbye to 151 delegates and watch the proportional share HRC will need to take go well beyond that which is possible. By the end of this, Obama might be in the position to simply accede to the FL and MI votes as they stand right now.