Progressives are right to want a primary opponent for Arlen Specter -- or at least to keep alive the possibility of one. As Chris Bowers notes, Specter has already cast two important votes against his party in his brief tenure as a Democrat, first on mortgage bankruptcy "cramdown" legislation, and then on the budget conference report, where he was joined only by Ben Nelson, Evan Bayh and Robert Byrd joined. Nor has Specter changed his position on the Employee Free Choice Act -- a measure which he had supported in past years but revealed in March that he would attempt to filibuster.
It's very early in Arlen Specter's career as a Democrat, and we will have to see his positioning evolves on other agenda items. Past party-switchers have tended to change their voting patterns in relatively meaningful ways following their conversions. It seems plausible that Specter would be reluctant to change his positions on issues which were already percolating on the Senate's agenda at the time of his party switch, and on which he had already articulated a position, but that he will become more liberal in the coming months.
For the time being, however, progressive Democrats have ample reason to be wary of Specter. Their problem is that Joe Sestak, the PA-7 Congressman who has refused to rule out a primary challenge, might not be any better from the standpoint of progressive policy.
In fact, it's plausible that he could be a bit worse. ProgressivePunch.org ranks Sestak as the 158th most progressive member out of 221 non-freshman Democrats, and notes that he's an order of magnitude or so more conservative than you'd expect of a Congressman from his Democratic-leaning district. Sestak's DW-NOMINATE score in the 110th Congress was -.287 on a scale that runs from -1 for extremely liberal to 0 for moderate; this is actually slightly more conservative than the score that we'd projected for Specter, which was -.303. The National Journal, moreover, found that Sestak took the liberal position only 63 percent of the time in the votes they tracked in 2007.
Nor would a primary challenge be without its downsides. For one thing, Sestak would have to give up his seat in the House in order to challenge Specter. Although the Democrat would still be favored in an open seat race in PA-7, which is 3 points more Democratic than the nation as a whole, giving up the incumbency advantage might reduce their odds of retaining the seat from, say, 95 percent to 75 percent.
There is also the possibility that Sestak would be more likely than Specter to lose to a Republican in November. I don't think this is a particularly strong worry for Democrats in this instance, since (1) Sestak is a charismatic and talented politician, (2) Sestak is a good fundraiser and (3) the probable Republican nominee -- Pat Toomey -- is much too conservative for Pennsylvania's electorate. Then again, it's precisely the fact that the Republicans seem inclined to nominate Toomey -- who may be borderline unelectable in November 2010 -- that raises the opportunity cost to progressives of nominating a moderate, whether it be Sestak or Specter.
Two other points to bear in mind, one of which makes a primary challenge more attractive for Democrats and the other of which has ambiguous effects. The first point is that the mere prospect of a credible primary challenge (and Sestak is a credible opponent) may have some "bluff" value, whether or not it actually succeeds and in fact whether or not it is actually executed upon. So long as Specter has reason to fear a primary challenge, it will push him toward the median of the Democratic electorate in Pennsylvania, which is probably about at the point occupied by Bob Casey Jr. (DW-NOMINATE score of -.401).
The second point is that Specter is 79 years old. Moreover, however exceptionally admirable his commitment to public service while undergoing chemotherapy for Hodgkin's lymphoma, he is not in the best of health. There is a fairly significant chance that he would not be able to complete his sixth term.
If Specter's term were to end early, then Pennsylvania's governor would pick a replacement, with a special election to follow at the next even-numbered November election. As Democratic incumbent Ed Rendell is term-limited, the governorship will be an open seat race, in which Democrats are probably favored but perhaps not overwhelmingly. Would Democrats prefer the safety of being "locked in" to Sestak for the next decade or two? Or would the they take the gamble a special election would represent as an opportunity to nominate and elect a more liberal candidate -- particularly if they had retained the governor's mansion?
What's clear is that a lot of these questions would be easier to resolve if Democrats were to select a more liberal primary challenger than Sestak. This is perhaps easier said than done, since the Democrats in Pennsylvania's congressional delegation tend to be quite conservative, and since many of the more prominent statewide officeholders are liable to run for governor instead. One intriguing possibility might be Franco Harris, the former Pittsburgh Steelers star who was an Obama delegate at the 2008 Democratic National Convention, and whose name recognition might allow him to be competitive on a relatively limited budget.