Virtually every media outlet in the country has descended onto Arkansas and Pennsylvania in recent days, trying to discern the meaning of tomorrow’s Democratic primaries, especially if incumbents Arlen Specter and Blanche Lincoln are headed for defeat (Specter could be, whereas Lincoln, at worst, is likely facing a runoff on June 8).
Their respective opponents, Joe Sestak and Bill Halter, are widely portrayed as lefty insurgents  in most outlets. The reality is not so simple. They are certainly insurgents—challenging the party establishments in both states—but they’re not exactly full-throated progressives. Sestak is probably the more progressive of the two, though he did support President Obama’s troop escalation in Afghanistan, whereas Halter is a deficit hawk who’s refused to say if he supports the Employee Free Choice Act, even though he’s been endorsed by the AFL-CIO and SEIU. Both Sestak and Halter would be relatively mainstream Democratic senators, I’m guessing, reflecting the hues of their states (light blue in Pennsylvania, solidly red in Arkansas). Both will face tough races in the fall and start out as underdogs, should they win their primaries.
Meanwhile, both Lincoln and Specter are under fire less because they’re moderates and more because they’ve been all over the place on big issues, appearing crassly opportunistic and devoid of conviction to both the left and right. As I wrote recently in my Nation piece  on Halter, there is no "purge " inside the Democratic Party comparable to the current purity tests within the GOP. Most moderate Democrats in Congress are not facing primary challenges this year and some frighteningly conservative Democrats  could win with substantial party support.
Halter is not an especially compelling candidate but his race is noteworthy because it’s forcing progressive groups to organize on politically conservative terrain in a tough election year, which they’ll have to do more of if they really want to change the Democratic Party and elect better Democrats across the map. The primaries on Tuesday are a key front in a much broader fight.
UPDATE: The Washington Post's Greg Sargent does a good job characterizing the differences  between the Democratic and Republican primaries tomorrow:
Halter and Sestak are mounting generally liberal challenges to their incumbent foes -- and despite this fact, there's still no equivalence between them and the ideological purgings we're seeing on the right.
That's because Halter and Sestak are trying to pull Lincoln and Specter in line with the Democratic mainstream, which neither represents. Lincoln and Specter are enjoying Dem establishment support despite being ideologically to the right of mainstream Dem positions.
Their challengers are fueled by an energetic grassroots effort to let the Dem establishment know this isn't acceptable. The Tea Party brigade, by contrast, is pulling candidates to the right of mainstream Republicanism. Therein lies the difference.