Updated

Let the political cannibalism begin.

Colleagues on Capitol Hill are being turned into rivals on the campaign trail, as newly drawn congressional districts force incumbents of the same party into primary campaigns against one another.

The phenomenon is a product of the once-in-a-decade redistricting effort occurring in state capitals across the country. State legislatures and other bodies, using the latest census figures, have been hard at work redrawing their state's congressional districts -- in some cases adding or eliminating them depending on the shift in population.

In states that are losing a seat or two, the result can be a game of ultra-competitive musical chairs, where the new borders cram incumbents who previously represented separate districts into a single district. Even a few states that aren't losing a seat have drawn maps that force incumbents to square off.

To date, both Republican and Democratic incumbents have been drawn into primary battles against incumbents of the same party. But Democrats of late have endured a string of bad news on the redistricting front -- most recently, the redrawn map in New Jersey pushed Democratic Rep. Steve Rothman into a primary battle against fellow Democratic Rep. Bill Pascrell. And in Pennsylvania, Democratic Reps. Jason Altmire and Mark Critz are expected to compete for the same seat while GOP districts are strengthened under a newly unveiled map.

"Republicans should be very pleased with the results of the ... redistricting," said David Avella, president of the Republican recruiting organization GOPAC.

In New Jersey, the Democrat-on-Democrat battle emerged Tuesday as Rothman announced his decision to campaign in the newly drawn 9th District.

Rothman already represents the 9th District, but the new map placed his home town of Fair Lawn into the new 5th District, currently represented by Republican Rep. Scott Garrett.

That could have been a tough fight, and Rothman ultimately decided to run in the new 9th, a decision Pascrell -- currently representing the 8th District -- had already made.

Both Democratic candidates are now stressing their deep ties to the north Jersey territory in a bid for primary votes.

Rothman noted that the majority of registered Democrats in the new 9th District reside in the old 9th District.

"I have represented the 9th Congressional District for the past 15 years and have lived here nearly my entire life. I look forward to continuing to represent this district," Rothman said in a statement, citing a long list of endorsements from local mayors and other officials.

But Pascrell's campaign, vowing to fight, said the congressman has represented the three largest communities in the new district "for the past 15 years" as well.

"Regardless of today's news, I am already out there working for the people of the new 9th Congressional District," Pascrell said in a statement Tuesday. "Everyone who knows me knows that I am a fighter who is ready for whatever may come. I will be as relentless in the election as I have been for my constituents. I do not know the meaning of the word quit."

While the map ensures at least one Democratic incumbent is going home before the general election in New Jersey, Republicans face a similar situation in other states.

So far, nearly a dozen primaries are expected to feature incumbents battling incumbents of the same party. Half are Republican-on-Republican races, including one in Louisiana and one in Arizona.

Among the other high-profile races on the Democratic side are one in Ohio, where Rep. Dennis Kucinich plans to challenge Democratic Rep. Marcy Kaptur in the primary, and in California, where powerful Rep. Howard Berman is running against Democratic Rep. Brad Sherman.

After a Republican wave election in 2010, the redistricting itself could account for much of the electoral change in 2012. An analysis in the Cook Political Report speculated that despite the general antipathy in America toward Congress, the "bulk of incumbent casualties" in 2012 could come simply from the redistricting process.

The process is not complete, and Republicans face a string of hurdles despite the recent turns in their favor. Texas officials are still battling in court over a new redistricting map, which in its initial form favored Democrats. Florida and New York also have not finished the redistricting process.

There is some speculation that the map in Florida could draw firebrand Republican Rep. Allen West into a primary battle against a fellow GOP incumbent.

In Pennsylvania, a Democratic Congressional Campaign Committee memo last week claimed the party remains in "strong position" to win seats despite the newly drawn map.

"Despite a closed-door process full of backroom deals and no public input - and slammed by community leaders, editorial boards, and ordinary Pennsylvanians -- Democrats remain competitive in Republican-held seats from Bucks County to Butler County," the memo said. "This map ensures competitive races across the state for the next decade."

But Republicans say the redistricting process as a whole is taking once-competitive seats off the table in November.

"The trend you're seeing of Republican seats being strengthened in the redistricting process is shrinking the playing field for House Democrats," said Paul Lindsay, spokesman with the National Republican Congressional Committee.