They called their caucus the Blue Dogs - in part, it is said, because as moderate-to-conservative Democrats in Congress, they felt choked to the point of death by the prevailing liberal orthodoxy of their party. Or, alternatively, frozen blue like a hound shut out of the house on a long, cold night.

Whichever etymology you prefer, though, the point is clear: The Blue Dog has become an endangered species.

Pennsylvania voters gave fresh evidence of that Tuesday, as two incumbent Blue Dogs lost primaries to opponents who attacked them from the left - particularly for having opposed President Obama's Affordable Care Act health-insurance overhaul.

But as much as anything else, congressional redistricting put these two dogs on the chopping block.

Rep. Jason Altmire lost in the redrawn 12th District in Western Pennsylvania to fellow incumbent Mark Critz of Johnstown, although Altmire had previously represented 68 percent of the new district's territory.

And in Northeastern Pennsylvania, Tim Holden, a 20-year House veteran and one of the first of the Blue Dogs, was slaughtered Tuesday in his reconfigured 17th District by a liberal trial lawyer, Matt Cartwright.

When Republicans in the state legislature redrew the district maps last year, they extended Holden's district northeast from his Schuylkill County base, packing in Democrat-rich Scranton and Wilkes-Barre. Eighty percent of the new district was terra incognita for Holden.

"My own core political beliefs are a much better fit for the new district," said Cartwright, who put $400,000 of his own money into the race and pounded Holden as too conservative for voting against Obama's health-care legislation and against environmental restrictions on the energy industry.

It didn't hurt that Cartwright had high name recognition from years of TV advertising for his law firm. and the League of Conservation Voters also poured money in to help him.

In the west, Critz was able to overcome Altmire on the strength of support from organized labor. Furious at Altmire's vote on health care, unions lined up against him in a district that had given Republican John McCain 54 percent of the vote in 2008.

"There were thousands of door knocks and tens of thousands of phone calls from labor," said Marc Stier of Philadelphia, who headed a grassroots group that drummed up support for the health-care changes. "Lesson No. 1 is that Democratic primary voters expect Democrats to vote with them."

Before redistricting, Altmire's and Holden's ability to break partisan ranks and work with Republicans was an asset, enabling them to survive in more conservative turf. In the new order, it became a liability.

Ironically, Holden survived a GOP attempt to kill him off in the last round of redistricting, 10 years ago; mapmakers had stuck him in a district that included parts of suburban Harrisburg and made him run against a Republican incumbent, George Gekas. But Holden triumphed, selling himself as a moderate who would represent the district, not his party.

Many Democrats, weary of hearing the GOP bash "Obamacare," are happy to see Holden and fellow dog Altmire lose to liberal challengers. But political scholars have long pointed out the perils of increasingly sophisticated mapping software and deeper data on voter behavior: a greater polarization in the House. As districts are tailored more narrowly to one party or the other, the committed partisans who turn out for primaries such as Tuesday's push candidates further right in the GOP or further left in the Democratic Party.

That, some fear, makes bipartisanship more unlikely, even as voters tell pollsters they crave it - and guarantees greater gridlock in Washington.

Stier, who also is a political scientist at Temple University, thinks that concern is overblown. He doesn't see this as the Democrats' version of the tea party effect on the GOP - Critz and Cartwright are not Delaware's Christine O'Donnell or Nevada's Sharron Angle, he said.

"We're not going to ask them to be loony leftists," Stier said Wednesday in an interview. "They'll still represent their districts. Mark Critz is not going to be some wild-eyed socialist."

But neither is he a half-froze hound.

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