It’s not easy being a centrist Democrat these days. In the April issue of The American Spectator‘s print edition, I point out that the Blue Dog Coalition in the House has actually done relatively little to rein in the Obama administration and their liberal congressional leadership. Now liberal pressure groups are mobilizing against the Blue Dogs and self-styled deficit hawks in the Senate Democratic caucus in order to keep it that way.

PACs like Accountability Now plan on supporting primary challenges to insufficiently liberal Democrats in 2010, acting like a netroots-connected progressive version of the Club for Growth. Other groups are up on the air with ads right now trying to get Democratic members of Congress to vote for Obama’s budget without trimming any of the spending. Even the Obama administration isn’t immune for liberal pressure, though Treasury Secretary Tim Geithner seems to be the fall guy for now.

A lot of the Democrats caught in the middle between conservative and liberal criticism face a Catch-22 politically: many of them could be vulnerable to primary challenges if they show too much independence from the party line, but their party-line voting makes them vulnerable to Republicans in the general election.