Iowa Rep. Jim Leach (R) seemed a natural to weather voters' antiwar sentiment this fall. His independent streak and moderate views had engendered the allegiance of his Democratic-leaning district for the past 30 years, and he broke with his party and President Bush in October 2002 by voting against the Iraq war.

Yet on Election Day, voters in Iowa's 2nd Congressional District ousted him in favor of an untested Democrat, a college professor -- a testament to the vulnerable position that Republican moderates found themselves in all year. Voters in Leach's district said that they respected him but that his party affiliation kept them from voting for him this year.

"He's a good guy, and he has integrity, and I think he has done a great job -- but he's still a Republican," said Jeremy Jackson, an Iowa City novelist.

With the defeat of Leach and several other Republican moderates Nov. 7, the Democrats' victory in the midterm election accelerates a three-decade-old pattern of declining moderate influence and rising conservative dominance in the Republican Party. By one measure, the GOP is more ideologically homogenous now than it has been in modern history. The waning moderate wing must find its place when the Democratic majority takes over in January.

"The irony of this election is that the public, in seeking change, has . . . weakened the center," Leach said recently. "In a sense, what has occurred is the strengthening of the edges of the parties."

Eight of the House's 20 most moderate Republicans lost their seats: Rob Simmons and Nancy L. Johnson (Conn.); Jeb Bradley and Charles Bass (N.H.); Michael G. Fitzpatrick and Curt Weldon (Pa.); Sue W. Kelly (N.Y.); and Leach. Also, moderate GOP Rep. Sherwood L. Boehlert (N.Y.), is retiring, and he will be replaced by Democrat Michael A. Arcuri, the Oneida County district attorney.

On the Senate side, the defeat of Lincoln D. Chafee (R-R.I.), a critic of the war who declined to vote for Bush's reelection in 2004, underscored the same trend.

By one measure, the 110th Congress will have the fewest moderates since the 19th century. This finding is based on an analysis of voting records by Keith T. Poole, a political science professor at the University of California at San Diego, and Howard Rosenthal, a New York University politics professor.

For the purpose of the study, a moderate is defined as someone whose votes consistently fall near the middle of the political spectrum on both fiscal and social issues.

The decline in moderates has had a greater impact on Republicans than Democrats. According to Poole's calculations, almost half of House Republicans were moderates 30 years ago, compared to well under 10 percent today. The professors argue that the decline of moderates in Congress has increased polarization.

To some moderate Republicans, the message of the Nov. 7 election was clear: The only path back to the majority is through the political center. With a small and shrinking membership, however, it is unclear whether the moderate wing will have much influence over the future direction of the GOP.

"There's a faction in the Republican conference that believes we lost the majority because we were never fiscally conservative enough, we were not socially conservative enough," Bass said. Indeed, over the last 12 years, the increasing conservative dominance in the GOP has mirrored growing Republican dominance in Washington -- first in Congress, then the White House.

Rep. Michael N. Castle (R-Del.), a prominent moderate, said he expects that his party will hold a series of internal debates next year about when to work with the Democrats and when to resist. Throughout the 109th Congress, moderates sided with the Democrats on such issues as stem cell research, drilling in Alaska's Arctic National Wildlife Refuge and overhauling immigration policy. Castle says moderates could build consensus on two issues likely to create fireworks between the parties: devising a new strategy in Iraq and reshaping lobbying.

Another moderate, Rep. Charles W. Dent (Pa.), said there is room for agreement within the GOP on tax and budget issues and on efforts to restore fiscal discipline and restrain the growth of government. "I think voters sent us a message" to get back to those types of issues, he said.

An important factor in the Democrats' victory in the midterm election was that independent and moderate voters abandoned the GOP in droves. Since the 2002 midterms, support for the Republican Party has declined seven percentage points among moderates and nine percentage points among independents, according to exit polls.

That trend was at play in Leach's eastern Iowa district, which includes rolling hills and farmland and high-tech plants and offices in Cedar Rapids. In 2004, Sen. John F. Kerry (D-Mass.) won 59 percent of the vote in the district, more than he won in any other Republican-held congressional district. Still, Leach, a Princeton graduate and one-time U.S. Foreign Service officer, defeated his opponent 59 percent to 39 percent.

During the past two years, as the president's approval rating plummeted and Iowa voters grew dissatisfied with Republican rule in Washington, Leach argued that the best way to alter the policies of the GOP and Congress was to elect more moderates to strengthen the political center.

That argument resonated with much of his district, but not in Iowa City, a heavily populated Democratic bastion and home to the University of Iowa. It was there that David Loebsack, a first-time Democratic candidate, carried a huge majority -- enough to unseat Leach, 51 percent to 48 percent.

"My opponent would have suggested the real change in Washington would be to get a check on Republican power," Leach said. "My opponent was clearly perceived as compelling in this election."

Loebsack said: "The party had moved away from him over the course of a number of years. His argument that he could bring them back to the center didn't wash any longer."

While Leach was going down, another prominent Republican moderate, Rep. Christopher Shays (Conn.), managed to hold on. Shays, unlike Leach, had supported Bush's war policies, despite strong antiwar sentiment in Connecticut.

Shays faced the same opponent he did in 2004, town official Diane Farrell, who had campaigned against him hard for two years. Shays knew he was a top target all year long, which gave him -- and Republicans in Washington -- time to raise and spend money to defend the seat. He also slowly altered his stance on the conduct of the war and called for the resignation of Defense Secretary Donald H. Rumsfeld.

Leach's vulnerability emerged late in the election. But when the Republican House campaign committee sent out two mailings attacking Loebsack, Leach, who had never run a negative campaign, told the party to stop the mailings.