House Rules Committee Chairman David Dreier (R-Calif.), center, announced Wednesday that he will not seek reelection at the end of this term. (Chip Somodevilla/GETTY IMAGES)

Rep. David Dreier (R-Calif.), a foot soldier in the Reagan and Gingrich revolutions who found himself politically drawn out of power amid the rising tide of the tea party, announced Wednesday that he plans to retire at the end of the year.

Dreier became the latest casualty in the decennial redistricting process that has pushed more than 20 House members into retirement, many of them longtime lawmakers, giving way to a new generation with a more partisan and confrontational style.

Dreier, the chairman of the influential but low-profile Rules Committee, mounted a rare public defense of Congress on Wednesday in asserting that he and House Speaker John A. Boehner (R-Ohio) had worked to reinstitute “regular order,” allowing each side more free-ranging debate than in the previous two decades.

“I am a proud institutionalist, and I believe that this institution is as great as it has ever been. . . . Both Democrats and Republicans can offer their ideas on the House floor,” Dreier said.

Unlike other retiring lawmakers who have denounced the seemingly endless gridlock in Congress — Sen. Olympia J. Snowe (R-Maine) cited unproductive partis anship in her retirement announcement on Tuesday — Dreier would gladly stay on Capitol Hill if doing so were a viable political option. A close ally of Boehner’s, his clout is at its peak.

However, California’s once in­cred­ibly stable congressional delegation was scrambled by a law that created an independent citizens commission to draw up the state’s 53 congressional districts without any regard for protecting incumbency.

The group did that, and then some. Over the past decade, when three wave elections swept across the nation, the Golden State was a fortress for incumbency, with just one seat changing hands between the parties. Now, after redistricting, the Rothenberg Political Report rates 11 House races in California as competitive, not including a pair of contests in which Democratic incumbents are facing off in primaries.

Some incumbents moved to other districts near their homes, while others are involved in their first competitive race in years. Dreier’s 26th Congressional District, in the foothills of the San Gabriel Mountains north and east of Los Angeles, was carved up in such a fashion that there was no political haven nearby, just heavily Democratic districts in every direction.

“David Dreier and Olympia Snowe retired for two completely different reasons,” said Nathan L. Gonzales, deputy editor of the Rothenberg Political Report, an independent political handicapper.

Before Dreier’s announcement, five other longtime California House members had said they would not seek reelection in November; another said he would run for San Diego mayor instead, and the rush to the gates began early last year when Rep. Jane Harman (D-Calif.) announced her immediate resignation to lead a think tank.

By this year’s end, those eight lawmakers together will have accumulated more than 180 years of congressional experience.

Nationally, redistricting is spurring a large number of retirements; 21 House members have decided to retire from politics, according to the Casualty List blog maintained by Roll Call, a Capitol Hill newspaper.

That number is expected to grow after the final district lines are drawn in two of the largest congressional delegations, New York and Texas, where political and legal infighting have delayed the process. The 112th Congress could produce the most outright retirements since the 104th Congress, in 1995-96, when the Republican takeover led by Newt Gingrich (R-Ga.) in the 1994 elections sent many long-term Democrats heading for the exits. All told, 33 House incumbents retired from politics that year.

The largest tally of retirements in modern times came in 1992, another period of deep public distrust of Congress amid scandals at the House bank and post office. Then, 52 House members retired outright.

The current House has been thrown into greater instability by a flood of retirements from the Senate, where Snowe became the 10th senator not to seek reelection. That’s the largest number of retirements since 1996, when 13 senators left.

Now, 11 more House members are giving up seats to run for the Senate, and that number is likely to increase, with as one or both of Maine’s House Democrats expected to run for Snowe’s seat. (In addition, John Ensign, a Nevada Republican, resigned in an ethics scandal and Dean Heller (R) was appointed from the House to take his seat.)

In his floor speech, Dreier acknowledged that he considered retiring two years ago, when Republicans were in the minority. Dreier, 59, first ran for office in 1978 as a graduate student at Claremont McKenna College but then was elected to the House in 1980 during the Reagan revolution.

He took the insider’s route to power as he slowly gained seniority on the Rules Committee, a below-the-radar panel that sets the terms of debate on every piece of legislation reaching the floor. By the time Dreier ascended to the top GOP slot in 1999, the panel’s influence had largely been usurped by the leadership, a practice that increased in the last years of GOP control and under the speakership of Rep. Nancy Pelosi (D-Calif.).

Boehner coaxed Dreier into staying by promising that, if Republicans reclaimed power in the 2010 midterm elections, he would bring back the freewheeling days when each side could offer dozens of amendments. It’s an experiment that’s still in progress. Some floor debates include a wide-open process, but the overall partisan tenor of Congress hasn’t been altered by the greater leeway for rank-and-file lawmakers.

“He has overseen a sea change in how the institution operates, from installing cameras in the committee’s hearing room to passing reforms that will long set the standard for transparency and accountability,” Boehner said in a statement.