Congressional District 6 candidates Ben Quayle, left, and David Schweikert, take part in a July 16 debate in Phoenix. (David Kadlubowski/AP)

No fight may be more vicious than a family fight, and in the family that is Congress, this election year features an extraordinary number of kin-vs.-kin battles in which lawmakers are forced to face off against one another for their political survival.

“These races tend to turn friends into enemies and bring out the worst in every political actor,” said David Wasserman, an analyst with the nonpartisan Cook Political Report, whose research shows that the 2012 election season includes more intraparty battles between incumbents than ever before.

Although the member-vs.-member contests won’t be decisive in determining control of the House, they disrupt friendships and fracture alliances. And by forcing voters to choose between compromisers and flame-throwers, old-timers and newcomers, they help determine the future direction of both parties.

Thirteen House races this year pit incumbents against one another — the most in at least two decades. Of those contests, 11 include members of the same party fighting for their party nominations and the chance to return to Washington.

In the coming weeks, no fewer than four primary battles will be waged between incumbents facing off in expensive and divisive races that have exposed deeper rifts in the parties.

In Arizona, for example, Republicans are taking sides in an Aug. 28 contest between two freshmen who took office in 2010 promising to cut spending and curb government reach and now argue over who has done a better job of keeping the pledge.

After a redistricting commission adopted a map that favored Democrats, GOP Rep. Ben Quayle, 35, faced a choice: remain in his dramatically altered and politically balanced district and face a tough reelection battle every two years, or switch districts to challenge fellow Republican freshman Rep. David Schweikert, 50, in a reliably Republican district.

Quayle put his house up for sale and rented another (owned by his parents, Schweikert is quick to point out) in Schweikert’s district.

Schweikert, a former treasurer of Maricopa County, insists that he’s more deeply rooted in the community.

“I had things in my refrigerator for longer than he had lived in the state” when he first ran, said Schweikert, who has received support from some tea party groups for his abrasive criticism of spending in Washington.

Quayle has been endorsed by Republican Sens. John McCain and Jon Kyl, and noted that he represents two-thirds of the voters in the new district.

“I’ve been battling for and fighting for conservative values and for changing the culture within Washington,” he said.

The tough fight is an “unfortunate situation,” he said. Schweikert called it “a disastrous waste of resources.”

Already this year, Dennis J. Kucinich’s congressional career was ended in Ohio at the hands of fellow Democratic Rep. Marcy Kaptur. Freshman Rep. Adam Kinzinger (R) ousted longtime congressman Don Manzullo (R) in Illinois.

In New Jersey, Rep. Bill Pascrell Jr. (D) vanquished Rep. Steven R. Rothman (D), a longtime friend, and in Pennsylvania, Rep. Mark S. Critz knocked off Rep. Jason Altmire, a fellow conservative Democrat.

Some incumbent battles are the result of redistricting, in which the party that controls the state legislature merges the districts of two incumbents in the opposing party.

Others result from nonpartisan redistricting commissions, which work to make congressional seats more compact and do so without regard for protecting incumbents from tough battles.

Still others came about as a result of basic math, as a state must lose a representative because of its shrinking population.

That’s the case in the month’s first two incumbent-on-incumbent battles, on Aug. 7 in Michigan and Missouri, where shrinking populations in Detroit and St. Louis forced lawmakers representing the urban cores and their suburbs into single, larger districts.

“I’d certainly rather be running against a Republican,” sighed Rep. Gary Peters (D), who has received strong union support and key local endorsements in his race against Rep. Hansen Clarke (D).

After his seat was wiped away in redistricting, Peters, who has served two terms, challenged Clarke, a freshman.

Peters, 53, and Clarke, 55, served together in the state legislature and once traveled together on a fact-finding mission to India. Both said they have a friendly relationship.

“I’m not concerned with him,” Clarke insisted. “My concern is more the well-being of the people I’m representing.”

The same day, Democratic Reps. William Lacy Clay and Russ Carnahan will square off in a district that encompasses St. Louis and its northern suburbs. Clay’s former district includes 70 percent of the new district, a distinct advantage.

Both congressmen are scions of well-known political families and their fight has been ugly, dividing in part along racial lines.

The month’s most notable race for exposing intraparty philosophical and generational rifts will take place the following week in Florida between Republican freshman Rep. Sandy Adams and House Transportation Committee Chairman John L. Mica. The Aug. 14 fight will mimic some of 2010’s battles between established Republicans and tea party favorites.

A former sheriff’s deputy elected in 2010 and now endorsed by former Alaska governor Sarah Palin, Adams, 55, has relentlessly tried to portray Mica, 69, as having the kind of free-spending ways that her 2010 freshman class was elected to correct.

“I think the question is: How do you expect someone who’s been here and helped create the mess, to be the one to fix the mess that they helped create?” she said.

Last month, Mica helped steer the first long-term transportation bill adopted since 2005 through Congress on a broad bipartisan vote. He insists that the measure represented a rare paring back of government, consolidating programs and streamlining construction and would have kept millions of construction workers on the job.

But 52 House conservatives, including Adams, voted against it because it contained too much spending.

Mica said he’s a fiscal conservative but makes no apologies for helping to build new bridges and roads in the Florida district.

“People want you to be advocates for the district, and I think I’ve been a strong advocate,” he said, noting that the projects he pushed were vetted for importance and backed by local officials.

Adding to the bitterness is that both think the race could have been avoided in a state growing so fast that it gained two seats in redistricting. The worst incumbent fights result when both challengers stubbornly think their opponent should have moved or run elsewhere and avoided the race altogether.

More of Adams’s constituents are in the new district. She says Mica first told her that she shouldn’t worry about a challenge, leaving her to assume that he would move and run in a new district north of their central Florida battleground. After she announced her candidacy, she said, he told her of his plans and insisted that she move.

He said she worked with the legislature to get a last-minute amendment to the maps to move district lines by a few blocks, placing her in his district. Both said they are firmly rooted in their neighborhoods and have no interest in running elsewhere.

Those who have gone through similar fights say there is little advice they can offer to help those still in the throes.

“You can endure it,” reassured New Jersey’s Pascrell, who has not exchanged more than pleasantries with the one-time friend he beat more than a month ago.

“Almost,” he added.