Americans are no longer flocking to the Republican Party as they seemed to be doing earlier this year. The shift in allegiance that pollsters found then has stopped, and the GOP has given back some of the ground it gained, a Washington Post-ABC News poll indicates.

According to the Post-ABC poll and other surveys, fewer people are describing themselves as Republicans than did so in the earlier months of the Reagan administration; more are saying they are Democrats and independents.

The new findings cast doubt on claims by some Republicans that the 1980 presidential race was a "realigning election" and that Republicans are about to replace Democrats as the majority party in the United States. At the same time, however, the Republicans are seen by the American public as better organized than the Democrats, better able to manage the government and more likely to control inflation.

What the new polls suggest and what some leading political consultants maintain is that the fortunes of the Republican Party are tied to President Reagan's popularity and his ability to cope with the nation's economic problems. When Reagan was highest in popularity, roughly until July, he seemed to be carrying some Democrats and independents into his party; they began characterizing themselves as Republicans.

Today the major national polls show Reagan with a lower popularity rating than at any time since his election, and the party allegiance polls also reflect that.

In the new Post-ABC News poll, 23 percent of those interviewed said they considered themselves Republicans and 39 percent Democrats, a margin of 16 percentage points for the Democrats. In a February Post-ABC News poll, 25 percent said they were Republicans and 32 percent Democrats, for a margin of seven points. Other polls, including those of the Gallup organization, the CBS-New York Times poll, and private political polls have shown the same pattern.

Robert Teeter, a pollster and campaign strategist for Republican candidates, said in an interview that talk of realignment is premature.

"We'll know after the 1982 and 1984 elections whether 1980 was a realigning election or whether it was just an instance of voters turning out a very unpopular Democratic president," he said.

Patrick Caddell, whose clients are mainly Democrats and is best known as Jimmy Carter's pollster, goes further and says he doubts that there will be any realignment. He says that "in the end, performance is going to matter," and that a negative public "verdict is moving in on this administration."

Since the 1980 presidential campaign, Caddell said, "people have been ready to go somewhere, and they would move on performance. Any realignment depends on success. But if that is the test, then the Republicans are getting off to a slow start."

The chief predictor of realignment has been the chairman of the Republican Party, Richard Richards.

Citing data from polls by Richard Wirthlin, Reagan's pollster, Richards has said frequently that the Republicans are "on the threshold of majority status" and predicted that the Republicans would win control of the House of Representatives in 1982.

While it is far too early for polls to offer meaningful findings about the 1982 elections, the Post-ABC News poll and a recent Gallup poll conclude that if those elections were being held today, the Democrats would in all likelihood retain their edge in the House.

Nevertheless, despite the fact that Democrats outnumber Republicans by five to three in the Post-ABC News poll, the Republican Party today is seen as more effective than the Democratic Party on a number of key national issues:

Fifty-three percent say the Republican Party "can bring about the kind of change the country needs"; only 44 percent say that of the Democrats.

Sixty percent say the Republicans would do a better job than the Democrats in controlling government spending; only 25 percent say the Democrats would do a better job.

By 48 to 27 percent, Republicans are preferred as the party better able to control inflation; by 53 to 31 they are seen as better able to hold down taxes.

According to findings by Teeter's firm, the Republicans held the advantage on most of these measures as far back as 1979, but not by such substantial margins. Many political observers, including Teeter, think the 1980 election was won by Reagan almost entirely because of the perception that Reagan could handle the economy better than Carter.

At the same time, however, comparisons between the new Post-ABC News poll and other polls taken a few months ago show the public becoming less certain that the Republicans are better able to handle economic problems.

A CBS-New York Times poll in April, for example, showed the Republicans doing better then than they are today in perceived ability to handle inflation and control federal spending.

In addition, the Democrats continue to be seen as the party that better serves many large sectors of the population and many aspects of the public interest.

By more than two to one, the Democrats are seen as better able to protect the Social Security system; by 47 to 31, the Democrats are viewed as more likely to see "that taxes are fair to everyone"; by 56 to 22 they are the party seen as better at "protecting the environment."

The Democrats are viewed as far more likely than the Republicans to protect the poor, to reduce unemployment and protect "equal opportunities for minority groups."

In the conventional wisdom, the American people are thought to regard the political parties as relics of the past and to see politicians and party leaders as corrupt, greedy and more interested in power than in what is good for the nation. The Post-ABC News poll finds some confirmation for that view, but also much to contradict it and make it seem simplistic.

Two out of three agree with the statement that "the two political parties do more to confuse the issues than to provide a clear choice on the issues." Two in three also say "the best rule in voting is to pick a candidate without paying any attention to the party label."

Half those interviewed feel the Democrats "care more about special interest groups than about the majority of the people," and slightly more than half say they feel that way about the Republicans. In all, about one person in five consistently rejects any positive description of both parties.

At the same time, a majority of Americans see both parties as having "the right goals for America," as being "forward looking, not old fashioned," and as tending not to be corrupt. Six in 10 Democrats and independents feel the local Democratic Party would encourage them if they wanted to become more active in party affairs; a similar six in 10 Republicans and independents say the same of the local Republican Party.

Most citizens, then, appear to have mixed, not one-sided views about the two parties. One question in the Post-ABC News poll asked people to rate each party from zero to 10, with zero meaning "the party and its leaders care only about their power" and 10 meaning "they care only about the best interests of the country." The average ratings were 5.2 for the Republicans, 5.9 for the Democrats.

The Post-ABC News poll, in which 1,516 people were interviewed from Nov. 17 to Nov. 22, also attempted to gauge why individuals change their party identification. In all, 27 percent of those interviewed said they had undergone shifts of affiliation, that they were Democrats or Republicans at one time but had either jumped parties or become independents.

The numbers moving from one party to the other just about cancel out. Six percent say they are Democrats who used to be Republicans, 7 percent say they are Republicans who used to be Democrats. The rest of the movement discerned was from one or both parties to the independent column.

The most often expressed reason for shifts in party affiliation is approval or dislike of particular candidates. One former Democrat, a 33-year-old professional singer and music director from Huntington Beach, Calif., said, "The choice of candidate for president made me change to Republican. I could not vote for Carter." More people cited disapproval of Carter as the reason for switching parties than any other factor.

Others trace their shift back in history. An Atlanta teacher's aide said she jumped from Republican to Democrat because of "the Richard Nixon mess that we as taxpayers are still paying for. I'll never forget that." Dissatisfaction with Nixon was the second leading reason for movement away from a party.

Many also cited Reagan, either as a reason for having moved toward the Republicans or away from them. One woman, a Hispanic nurse from Miami, says she is now a Republican and no longer a Democrat because she believed "Ronald Reagan when he said he would put America back on its feet. The Democrats screwed things up."

But a 22-year-old black college student from Washington, D.C., said he had been a Republican but switched to the Democrats because of Reagan. "I understand what Reagan says he wants to do. He offers hope and faith, but he is cutting all these programs."