LOS ANGELES >> Democrat-on-Democrat warfare has returned to the San Fernando Valley.

After a tiff over the redrawing of their adjacent congressional districts a decade ago, Rep. Brad Sherman and his fellow California Democrat, Rep. Howard L. Berman, both won re-election and went on to work cordially — if not lovingly — on home-state legislation to help the region they both serve as House members. Evidently, those days are over.

This redistricting go-round, the two popular, veteran congressmen find themselves running for the same seat in the most high-profile of about a dozen primary races that mark the decennial return of internecine fights that can severely rupture relationships of House members who sit on the same side of the aisle.

"My guess is we probably don’t communicate as much as we used to," said Berman about his interaction with Sherman because of a race that both men would have preferred to do without.

"Perhaps the friendship has been a bit impaired by the situation," said Sherman, agreeing that the campaign has taken a toll on their relationship and caused spats over issues, like Iranian sanctions, "that wouldn’t have happened six months ago."

So far this election cycle, there are roughly an equal number of primaries between sitting lawmakers divided among Republicans and Democrats. Many members continue to weigh options like moving to other districts, and some states have yet to draw their maps, leaving the situation in flux.

The Los Angeles-area race is one of three such potential matches in California; others are occurring in Illinois, Louisiana, North Carolina and Ohio, among other states. Incumbent lawmakers of different parties will also be squaring off in November as they compete for shrinking congressional turf in places like Iowa and Ohio.

This year’s face-offs between incumbents are expected to be far more complex than in the past, as the way to reach voters has splintered into micro-genres unheard of in previous contests. The warfare is especially intense in places like Illinois, where seats have been lost to other states and the desperation among members to keep their jobs is palpable in their gloves-off punches at their own party mates.

"Redistricting causes chaos throughout the country," said Nathan L. Gonzales, the deputy editor of The Rothenberg Political Report, a nonpartisan political newsletter. "The overall level of sophistication in these campaigns and the money spent will be more than 10 years ago."

The contest between Sherman, elected in 1996, and Berman, elected in 1982, is expected to be the most expensive House primary in the nation, owing in part to the costly media market that is Los Angeles, and California’s new open primary system, in which the two top votegetters — quite possibly Berman and Sherman — face off again.

While the new "super PACs" that funnel anonymous money into races have so far been rare in House primaries, expect to see them here. As of the end of September, Berman had $2.2 million in cash on hand, and Sherman $3.7 million.

"This is going to be the battle royal," said Eric Bauman, chairman of the Los Angeles County Democratic Party. "Both of these guys will be extremely well funded with top-flight campaign teams. They both have records to talk about and accomplishments to talk about. But ultimately it is about defining themselves, and that is where money comes in."

A few dozen freeway miles south into the Los Angeles basin, Rep. Janice Hahn, a Democrat who won a special election this year, will be stuck facing off with Rep. Laura Richardson. However, State Assemblyman Isadore Hall III, who is a strong fundraiser, has joined the race and could send them both packing.

Also among Democrats, Rep. Dennis J. Kucinich has found himself locked in battle with Rep. Marcy Kaptur in Ohio, a prospect that briefly led Kucinich to consider relocating to Washington state.

On the Republican side of the ledger, a freshman Tea Partier, Rep. Jeff Landry, and four-term incumbent Charles Boustany are in a closely watched intramural contest in Louisiana. Landry is already trying to play his Washington outsider card — though he may be considered an insider now — touting his votes against House leaders.

Another expected Republican match-up could pit freshmen Reps. Joe Walsh and Randy Hultgren in the Chicago suburbs. Although Walsh, a fiscal hawk, is considering staying in his Democratic-leaning district, he has already started a spirited campaign against his fellow Illinois freshman, accusing him of not being as conservative as he. Walsh’s wings have been clipped a bit, however, by revelations that he owes his ex-wife tens of thousands of dollars in child support.

The Sherman-Berman race — or the Berman-Sherman race, depending on one’s loyalty — is one that has riveted political insiders in Los Angeles. About 60 percent of the new district’s voters are from Sherman’s current district. His ubiquity — 150 town hall meetings, weekend concerts in local parks and e-newsletters stuffed into constituents’ inboxes — is his claim to fame here.

"Brad’s done a great job at his community outreach meetings," said Kory Peterson, 53, an undecided Granada Hills voter, at a parade in the neighborhood Sunday.

Sherman was back in Washington and unable to march among the bejeweled reindeer, but the California Democrat left nothing to chance. His 82-year-old mother, Lane Sherman, her hair and lipstick an impressive matched set of fire-engine red, rolled along the route in a 1952 Chevy, bellowing into a bullhorn, "Happy Holidays from Congressman Brad Sherman, my son!"

But Berman, who is more known for his legislative career and expertise in foreign policy, has the backing of the majority of his fellow congressional Democrats from California and some of the fundraising might of Hollywood.

"I have contributed to him and supported him for many, many years," said David Geffen, the Hollywood mogul, who was one of the hosts at a fundraising party for Berman that brought in about $1.6 million. "I like him personally and would hate to lose him in the House."

Both lawmakers are popular among Jewish voters — who are sprinkled in large numbers throughout the diverse new district.

Most Democrats here find the Democratic face-off a sad state of affairs.

"The bottom line," said Bauman of the Los Angeles Democratic Party, "is that the Democratic community and the Jewish community and California all lose here,B because a top-flight Democrat is not going back to Washington."

Berman encouraged Sherman to run in a nearby district in Ventura County to avoid the race, but Sherman would have none of it.

"If Howard thinks the best approach to winning the Ventura County seat is for someone to move there from L.A.," he said, "he ought to do it."

Both men express confidence they can win.

"There is a lot I still want to do here," Berman said. "I deliver in terms of my constituents and in terms of legislation back here and projects for my district in a way that will cause me to win the election."

Sherman said that he wished it had not reached this point, but he was plunging ahead.

"I’ve had close races in the past," he said. "They’ve been against a Republican. I like that better."