If Sen. Arlen Specter (D-PA) faces a primary challenger in the Pennsylvania Senate race next year, it's likely to be from Rep. Joe Sestak (D-PA). Sestak was considering a run before Specter switched parties, but that development undoubtedly turned his game plan on its head. Now, winning the Democratic nomination will be much harder than it would otherwise have been, but he has a much more compelling raison d'être. Now, if he runs, he'll be fighting a man who he says is an unreliable steward of Pennsylvania's interests, and a weak choice for Democratic voters who want a real Democratic senator.

Sestak told me that he'll be inclined to primary Specter if he doesn't prove a reliable Democratic vote on all major Democratic issues. But how reliable is Sestak?

Pretty reliable on the whole. According to the Washington Post votes database, Sestak has voted with his party 97.8 percent of the time.

But he has bucked his party on a number of key votes, particularly on national security issues.

He voted, along with party leadership, but against the bulk of members, against last year's FISA Amendments Act, which vastly expanded the government's legal wiretapping authority, and provided, for all intents and purposes, immunity to the telecommunications companies that abetted the Bush administration's warantless eavesdropping program. His statement on the passage of that bill is here.

One day before that, he voted against most Democrats to approve yet more war spending for Iraq. "Whatever one thinks about the decision to go into Iraq and the consequences of this war, it is imperative that we support the men and women who wear the cloth of this nation and are willing to sacrifice everything to defend our country and the ideals we represent," Sestak said at the time. "I will always vote to fund our troops--until we can set either a date certain or they are fully redeployed from Iraq, which I support--and will continue to fight to assure that they are given every opportunity to succeed when they return to civilian life through measure such as the GI Bill."

Sestak consistently voted against House measures to end the Iraq war via the appropriations process. When I interviewed Sestak earlier this week, he criticized Specter for supporting it.

More recently, Sestak voted against two measures in the House--the End Government Reimbursement of Excessive Executive Disbursements Act and the Pay for Performance Act--aimed at limiting and recouping bonuses paid to financial services employees and executives. Sestak said the measures were, respectively, unclear (and therefore likely to result in expensive lawsuits) and too broad (and therefore likely to unfairly target the recipients of reasonable bonuses).

Sestak has voted against Democrats on other issues, too, and I've asked his staff to comment on a handful of those votes. Unlike Specter, Sestak voted for the Obama budget earlier this year.

Recently, Nate Silver weighed the pros and cons for progressives of a Sestak candidacy. He noted that Sestak isn't a particularly liberal Democrat, and that party switchers like Specter tend to change their voting patterns in a fairly reliable way. He suggested that Sestak could in theory prove to be a less progressive senator than Specter, and made the point that if Specter wins, he might, for health reasons, not be able to complete a full six year term. That would allow the Democratic governor to appoint somebody to Sestak's left, whereas if Sestak wins, he'll, in all likelihood, be locked in for a full term or more.

All points worth considering, but it's important to recall that as long as Sestak's running, or threatening to run, both men will feel the incentive to move left. That's good news for progressives on its own, and even better if the brinksmanship results in a sort of political positive feedback loop, where each candidate does his best to prove himself more liberal than the other.

Late update: Sestak's office sends along an explanation for a number of votes.

1. Congressman Sestak voted for HR2642 and HR2764 [Veterans and State Department appropriations, respectively] because they included emergency supplemental funding for the troops and funding would have otherwise been cut off well prior to their being able to redeploy safely. Congressman Sestak made it clear that he opposed the War in Iraq, submitting a bill soon after arriving on Capitol Hill that called for a "date certain" redeployment from Iraq in one year. This would have provided full funding until the day all troops were out, in one year. However, after the "surge" of increased troops, the Congressman knew that a one-year timetable was no longer realistic, and submitted HR3863, which called for redeployment after 15 months. With his military background, the Congressman understood that redeployment is the most dangerous of military operations, and takes time to execute in a safe and effective way. As long as our troops were in harm's way, the Congressman voted to ensure the funding for their safety, without which they would have been potentially put in greater danger. In short, he had a bill for full funding until the day they got out. To have voted against HR2642 and HR2764 would have cut their funding off before they had time to redeploy safely.

2. As stated above, soon after joining Congress, Congressman Sestak submitted HR960, which called for redeployment from Iraq after one year. However, after the "surge," the Congressman knew that a one-year timetable was no longer realistic, and submitted HR3863, which called for redeployment after 15 months. Therefore, as a result, after the "surge," Congressman Sestak voted against HRes818, because he wanted HR4156 to be sent back to committee and changed to include a more practical 15-month timetable for redeployment instead of the one-year timeframe that was in HR 4156. However, when the bill came to a vote, the Congressman decided, on balance, to vote in favor.

3. Congressman Sestak has voted several times against adjournment because he believes that Congress, when faced with pressing situations, should stay and complete its work.

For example, he voted against adjournment in July 2008 because the Nation faced a serious energy crisis. This Nation has not had an energy policy for 30 years and with the issues of high gas prices and over-reliance on foreign countries for energy, Congress had the opportunity and momentum to remain in session and work on it. In that moment of crisis Congressman Sestak believes they should have remained in session.

This past February 11, Congressman Sestak opposed going to March recess because the Nation was in middle of an economic crisis. Congressman Sestak believes that, having not yet passed the economic stimulus bill, Members of Congress should have stayed to work and not left the front lines when so many of his constituents were struggling to pay their bills and rightly fearful of losing their jobs.