The centrist Democrats known as "Blue Dogs" sometimes say they've been "choked blue" by the liberals who dominate their party. But now it is Republicans and redistricting that are threatening to cripple this once-influential group of House members.

From The Wall Street Journal:

The centrist Democrats known as "Blue Dogs" sometimes say they've been "choked blue" by the liberals who dominate their party. But now it is Republicans and redistricting that are threatening to cripple this once-influential group of House members.

The ranks of the Blue Dogs—known for their fiscal conservatism and the central role they played in the 2010 debate over health care—have already fallen from 54 members before the last election to 25 today. With the defeat of two leading members in Tuesday's Pennsylvania primaries, Reps. Tim Holden and Jason Altmire, the group has absorbed another blow.

Adding to their woes, five Blue Dogs have announced they're leaving the House this year, in several cases after Republicans redrew their districts.

Still others face tough elections in November. Rep. Leonard Boswell (D., Iowa) is pitted against a fellow incumbent, Republican Rep. Tom Latham. Rep. Jim Matheson (D., Utah) is being challenged by Saratoga Springs Mayor Mia Love, a rising star who would be the first black Republican woman in Congress.

It's always possible that new Blue Dogs will be elected this year to replace those departing. But House districts are increasingly drawn to favor conservative Republicans or liberal Democrats, making it harder for centrists to win office.

Rep. Mike Ross (D., Ark.), co-chairman of the Blue Dogs, who is himself retiring, blamed redistricting and polarization for the group's struggles. "Folks in the middle get attacked from the far right and the far left, because we are not driven by the all-too-common ideological partisan divide," he said.

Another Blue Dog leader, Rep. Heath Shuler (D., N.C.), who's also stepping down, said the group's dwindling numbers make legislative compromise harder.

It wasn't long ago that Blue Dogs were seen as Washington power brokers. In 2006 and 2008, Democrats recruited conservative candidates in swing districts. The result was the election of 19 new Blue Dogs, giving the group a voice on issues such as health care and energy.

Blue Dogs forced President Barack Obama two years ago to drop his goal of including a government-run insurance plan in his health-care overhaul, which eventually won a House majority and became law.

In recent months, Blue Dogs have taken a leading role in trying to forge a deficit-cutting deal with both spending cuts and tax increases.

Republicans say the Blue Dogs' fading fortunes reflect a Democratic shift to the left that is making the party more hostile to centrists. Rep. Lynn Westmoreland (R., Ga.) said that when Democrats held a 257-178 House majority after 2008, they didn't need the Blue Dogs and passed a series of liberal bills that turned off voters in swing districts. "The Blue Dogs lost because the Democrats had too big of a majority," he said.

Democrats scoff at this, saying their party is far more open than today's GOP to a spectrum of views. "I don't think there's any doubt that the House Democratic caucus is still the most diverse body of legislative representatives you will find," said Rep. Xavier Becerra (D., Calif.).

Messrs. Holden and Altmire were defeated Tuesday by more liberal Democrats, rather than losing to conservative Republicans, the fate that met many other Blue Dogs. But their defeats did result from aggressive redistricting by Pennsylvania Republicans.

As part of their effort to boost GOP incumbents, party leaders moved thousands of liberal voters from the district of Rep. Lou Barletta (R., Pa.) into Mr. Holden's district. Mr. Holden's centrist record was unpopular among these voters, who on Tuesday nominated attorney Matt Cartwright.

Mr. Altmire was forced by redistricting to run against another incumbent Democrat, Rep. Mark Critz, who was aided by unions that were upset by Mr. Altmire's record, including his vote against Mr. Obama's health-care law.

Mr. Cartwright is now favored in November, while Mr. Critz faces a tougher road in a Republican-leaning district against GOP nominee Keith Rothfus.

In a twist, the Blue Dogs did see recent growth from one quarter, as Rep. Larry Kissell (D., N.C.), facing a difficult re-election battle, joined the group in the past few days. Redistricting gave Mr. Kissell's district more Republicans, and the Blue Dog label could help with conservative voters.