Setting tonight's guidelines: Will Pennsylvania's primary be tighter than Mississippi's special?

This is the third Election Day in which Barack Obama has been in a position to bury Hillary Clinton; he came short on January 8th in New Hampshire and failed to do so again in Ohio and Texas on March 4th. From now on, of course, every Election Day could mean instant death for the Clinton campaign who has no room for error all the way until June 3rd. To continue using the metaphor of a tennis match, Obama has quadruple match points -- and some of them (North Carolina, Oregon) are on his serve.

Clinton is hoping that a solid showing today in Pennsylvania will help her finally get some momentum and head into the May contests in a strong position. Polls over the past few days suggest that Clinton is heavily favored to win, but the margin of victory is more unclear; few polls are showing anything more than a 10% lead and the key to tonight's results seems to reside in the turnout of groups that typically favor Clinton (blue-collar whites, Catholics).

The issue, of course, is defining what a strong showing would consist in. There are, as was the case in Ohio and Texas, three separates issues to consider: (1) What result does she need to stay in the race? (2) What result would make her claims of victory credible? (3) What result would change the fundamental dynamics of the race? It's worth addressing them in turn.

(1) What Clinton needs to simply stay in the race is perhaps the hardest question to answer because it fully depends on Clinton's determination; only she can decide to bring an end to the race by dropping out. It would be very difficult for her to stay in if she loses Pennsylvania; but what about a narrow victory (say 3-4)? That would be interpreted as a very weak result for the Clinton campaign; but would she pull the plug?

(2) The threshold of a credible victory is fairly clearly defined: Clinton needs a double-digit victory, or at the very least a very high single-digit lead. Under this "Ohio threshold," the Obama campaign will claim to have exceeded expectations and pride itself on a moral victory. Above it, it will be nearly impossible for Obama to spin this into a decent showing and questions will only build as to why Obama is unable to close this off.

(3) To change the fundamentals of the race, Clinton needs much more than a double-digit victory. Her surprising comeback in Ohio, after all, allowed her to stay in the race but it did not alter the primary's basic dynamics. Backed into a corner, Hillary needs a dramatic showing to get some air -- and that involves exceeding expectations with a comfortable double-digit victory of at least 15% and she also needs exit polls to show Obama has come in very weakly among white blue-collar voters. Even then, of course, Clinton would remain the heavy underdog but she would finally have succeeded in moving the race meaningfully.

These measures might seem unfair, of course, considering that Obama has massively outspent Clinton; not to mention that, considering how close she is from the exit, it is remarkable that Clinton is still standing -- let alone in a position to win a primary as important as Pennsylvania's. But this year's contests have been defined by demography: Both Democratic candidates have their core constituencies, and neither has made significant inroads in the other's base groups. If anything, African-Americans are more determined to vote for Obama than they were in January and downscale whites are looking more solidly in Clinton's camps than they were at the end of February.

In this trenchware warfare, the slightest weakness in either candidate's core groups have cost the candidates greatly (Clinton in Wisconsin) but unusual displays of strengths have yielded great rewards (Clinton in NH and OH, Obama in SC). If Clinton blinks again, it could prove fatal to her campaign; but if she keeps her groups mobilized today it could be very damaging to the Obama campaign.

Mississippi's 1st district: Also today, voters in the MS-01 are going to the polls for the first round of the special election to replace Rep. Wicker. This has become a surprisingly contentious election considering how red the district is; the NRCC have dumped almost $300,000, more than twice the amount spent by the DCCC, testifying to how panicked the GOP has become at the prospect of unexpectedly losing a conservative Southern seat.

The two main candidates are Democrat Childers and Republican Davis -- but the ballot will feature 6 candidates total, including two important politicians (one from each party) who lost to Childers and Davis last month in the primary for the actual November election. They are not campaigning for the special election, but their presence on the ballot makes it almost impossible for either of the main candidates to get elected tonight.

If no one reaches 50% today, a runoff will be held on May 13th; but tonight's results are certainly very important for they will tell us how strong a base vote Democrats have and how much bitterness there still is among Republicans after a very contentious primary. Depending on the answer to these questions, the DCCC might decide to invest more in the district or to give up the fight -- and that will go a long way toward determining what will happen in the runoff.