Specter: A known commodity and the state’s most dominant political figure over the past 30 years. Others have come and gone, but with a lot of luck in many close elections, Arlen remains. People don't love him, but many grudgingly respect his seniority. His vulnerability is spending – and if the primary and general races are defined by earmarks and Washington’s culture of corruption, he’s in big, big trouble: He is a master of inside-politics, willing to spend, and never has been in the lower-taxes, smaller-government crowd.

Toomey: A credible, younger challenger with a clear message on earmarks and spending. However, he is not a great orator and has not proven himself able to motivate people beyond the front-bench believers who are with him because he isn't Arlen.

Think of a packed church on Easter Sunday, when everyone in the congregation sees themselves as “saints”:

Those up front are there every week for every service; Toomey has already captured them because he isn't Arlen.

Those in the mid-pews are excited to learn more about the guy, but Toomey has not yet convinced them. (This is the key to his poor performance in Central Pennsylvania's conservative “T” when he ran against Specter in 2004; he needs them to win, and they want to follow him – but they need a reason.)

Those in the back pew are only there because it’s a holiday; they’ll vote in 2010, and Arlen has their votes because he’s the known commodity.

Shapiro: Clearly an overall good candidate, he appeals to Arlen's base for money and votes and could be seen as a newer model at a time when voters are looking for a trade-in. He appears to have only one problem: He’s a member of the Pennsylvania House, the absolutely worst recruiting spot right now. But Shapiro has done a good job of defining himself as a reformer and an outsider in the statehouse culture of never-ending corruption.

Torsella: A proven fundraiser, known in the Philly establishment as an all-around good guy, but he ran a lackluster, mismanaged congressional primary against Rep. Allison Schwartz that he should have won. At this point, it’s hard to see him beating Shapiro.

One wildcard is State Auditor General Jack Wagner, the only Democrat who won more Pennsylvania votes than Barack Obama in November 2008. Wagner getting into this race is plausible, but the smart money still is on Shapiro.

Can any of these potential challengers take out Arlen with an anti-spending, anti-corruption message? Will the stimulus vote be a plus or a minus for Arlen? How much will Obama help the primary-winning Democrat, since Specter has handed him several key votes in the U.S. Senate?

All of these unknowns will make this 2010 race of particular national interest.