WASHINGTON, Nov. 14 - The once mighty Southern Democrats are an increasingly endangered species on Capitol Hill.

In the new Congress, only 4 of the 22 senators from the 11 states of the old Confederacy will be Democrats, the lowest number since Reconstruction; as recently as 1990, 15 of those Southern senators were Democrats. In the House, the Democrats suffered smaller but still significant losses in Texas, where a Republican redistricting plan took down a group of veteran lawmakers, including the paradigmatic Southern conservative: Representative Charles W. Stenholm, a 13-term deficit hawk and longtime leader of the Blue Dog Democrats, a group of centrists in the House.

This moment has been a long time coming. Ever since the national Democratic Party fully embraced the cause of civil rights 40 years ago, shattering its hold on the so-called solid South, Republicans have been making steady inroads among culturally conservative white voters in the region. But the acceleration of this trend is important for the next Congress: some of these Southern Democrats, along with Northeastern Republicans, were among the last remaining lawmakers in the political center of an increasingly polarized House and Senate.

Their dwindling numbers, analysts say, could intensify the divisions on Capitol Hill. The retirement of senators like John B. Breaux of Louisiana means "you're losing moderate Democrats who could work across the aisle," said Larry Evans, a professor of government and an expert on Congress at the College of William and Mary. "And what you're left with is a more polarized Senate." That could mean "more partisan conflict, more infighting, the minority being more reliant on the filibuster and the hold," Professor Evans said.

Senator Olympia J. Snowe, the Republican moderate from Maine, said she had found a "natural alliance" with many of the centrist Southern Democrats over the last 20 years in the House and Senate, on issues like the budget and health care. In the Senate, she and Mr. Breaux worked together on Medicare legislation, including last year's overhaul, which created new prescription drug benefits for the elderly.

"It will mean searching out different majorities on some of these issues," Ms. Snowe said.

This could also have important implications for Mr. Bush's domestic agenda. He needs bipartisan support to achieve major changes in Social Security, for example, but two Democrats considered most likely to work across party lines for entitlement "reform" will not be there: Mr. Breaux and Mr. Stenholm. Some Democrats on Capitol Hill said last week that the Republicans, who campaigned hard against Mr. Stenholm, had perversely cost themselves a potential ally.

Mr. Breaux, who first came to Capitol Hill as an aide to Representative Edwin Edwards and won his boss's old House seat in 1972 at the age of 28, was an aggressive dealmaker on taxes, Medicare, health care and an array of other issues. "I loved being in the middle," Mr. Breaux said last week, one of his last weeks in office. "I loved to do the deal, and everybody knew it."

Merle Black, an expert on Southern politics at Emory University in Atlanta, noted that for much of the 20th century, Southern Democrats used their clout and their safe seats on Capitol Hill to "defend the South against civil rights legislation." But after Lyndon B. Johnson and the national Democratic Party pushed through the major civil rights legislation of the 1960's - a move that Mr. Johnson is said to have said at the time would cost his party the South - Democrats in the region became adept at the art of biracial politics.

Southern Democrats of Mr. Breaux's generation were canny coalition builders. In the 1970's and beyond, they continued to exercise substantial power in both the House and the Senate, rising by virtue of their seniority to run many of the major committees. They began to suffer some of their heaviest losses in the Reagan era, as the Republican realignment gathered steam in the region.

Mr. Breaux, waxing somewhat nostalgic, noted that his predecessor in the Senate was Senator Russell B. Long, longtime chairman of the Senate Finance Committee and son of Huey Long. "He could get more done in the afternoon, after work over a bottle of bourbon, talking with the other side," Mr. Breaux said. "He could put a deal together, and I would argue the country wasn't any worse off for it."

The retirements of Mr. Breaux, as well as Senators Ernest F. Hollings of South Carolina, Bob Graham of Florida and Zell Miller of Georgia, created a critical opportunity for the Republicans in the region. Mr. Miller was a relative latecomer to the Senate, but Mr. Breaux and Mr. Graham had held their Senate seats since 1986, and Mr. Hollings since 1966; incumbency, and the familiarity it brings, had long protected Democrats from the increasingly conservative bent of Southern voters.

Republican strategists said they had many other assets in the South this year, including President Bush's formidable coattails in the region and a strong message of patriotism, individual freedom, lower taxes and moral values.

"Our candidates were the ones people in the South feel comfortable with," said Senator George Allen of Virginia, chairman of the National Republican Senatorial Committee. In Georgia, Louisiana, North Carolina and South Carolina, the Republican Senate candidates carried two-thirds or more of the white vote, according to surveys of voters leaving the polls.

Professor Black of Emory University said: "Here's the problem Democrats face in the South: They've lost the white conservatives. They're now losing the younger, white moderate voters, who are now much more likely to be Republican than Democrat."

Democrats can still win in many Southern states without a majority of the white vote, because they usually get such strong support from blacks, Professor Black noted. But they cannot prevail against such lopsided losses among whites.

Mr. Allen said the results in the South exceeded even his bullish predictions. He was not confident, he said, that Representative David Vitter, the Republican candidate to succeed Mr. Breaux in Louisiana, would be able to get more than 50 percent in a multicandidate field and avoid a runoff. But he did, and he became the first Republican ever elected to the Senate from Louisiana.

Glen Bolger, the pollster for the Republican Senate candidates in the Carolinas, reflected his party's new confidence: "There will be exceptions to the rule, but the rule is going to be Republicans representing most places in the South."

Others disagree, including Senator Blanche Lincoln of Arkansas, one of the few Southern Democrats left standing in the Senate. "We may be few in numbers at this juncture, but I think we play a vital role, as the moderates, the centrists, who try to bring things back to the middle," Ms. Lincoln said. "Because if you look at the polls, the majority of Americans are still in the middle. They're not at the extremes."

And Representative Marion Berry, another Arkansas Democrat, said his party's House losses in the region were due to a "very skillfully done gerrymandering job" in Texas, not some underlying political trend. "My goodness, Charlie Stenholm is the most conservative guy around," he said sadly of the defeated Texan.