Does swing to GOP extend beyond Reagan popularity?

Exuding good cheer and feeling that he's on a roll, President Reagan is forecasting a ''historic electoral realignment'' of political parties, as millions of Democrats abandon their traditional base to vote Republican in 1984.

Is such a realignment in the wind? Or are the Republicans simply whipping up campaign momentum?

As Mr. Reagan embarks today on a final five-day, 10-state campaign blitz across the United States, surveys do show that the Republicans are edging closer to the Democrats, who still remain the majority party. But election experts caution that the apparent voter shifts could be due more to support for the President and to the Reagan coattail effect than to a basic and enduring change in political attitudes. That is something that will have to be analyzed carefully after election day and will not become evident until the elections of 1986 and beyond.

''The parties are closer than we've seen in a long time, closer than in the rout of 1972,'' says Andrew Kohut, president of the Gallup Organization. ''But I do not see a fundamental ideological shift in the climate of the country. It's more a case of 'If nothing's broke, don't fix it.' On many issues people disagree with Reagan, yet they support him.''

Political analysts suggest that GOP talk about realignment is designed primarily to make American voters believe that the race is over. They see Reagan not as a Franklin Roosevelt, who did lead a party realignment, but as a Dwight Eisenhower, who won a second term as a popular president. After the election, a fierce struggle is expected among the different wings of the Republican Party for control of its ideological future. The Democrats, too, if their standard-bearer loses, will be searching for new directions.

''I don't think the country is becoming more conservative,'' says Frank Mankiewicz, a longtime Democratic operative. ''When you do a poll on the man, Reagan wins. But if you do a poll about ideas, Mondale wins.''

Reagan strategists themselves say they are not striving for a long-term party realignment but merely to improve the Republican presence in Congress - keeping the Senate and picking up seats in the House. ''We just want to get back to the kind of situation the President had in 1981, where he could get his programs through,'' says independent consultant Charles Black.

At the presidential level, however, there has already been a shift toward the Republicans in recent elections. The question is to what extent this election will signal appreciable change at the congressional and local levels.

The Roosevelt shift of 1936 ''will not happen, but there has been a slow realignment over 20 years, and this election will give it impetus,'' says William Schneider of the American Enterprise Institute. ''The Republicans have been making gains at the presidential and Senate levels, so this is a drift to the Republicans, but not an earthquake.''

Mr. Kohut says that in the present congressional races 48 percent of the voters say they are likely to vote Democratic and 46 percent Republican. Four years ago the figures were 51 percent Democratic and 42 percent Republican.

Going back to the election of 1972 at this same point in the campaign, 47 percent said they would vote Democratic in congressional races and 42 percent Republican. But, significantly, the gap widened again in 1976 - to 55 to 36.

''The essential question is why the gap bounced back after 1972,'' Kohut says. ''Was it because of Watergate or because people returned to their traditional inclinations?''

The present gains for the Republicans are taking place in part among young people under the age of 25, a shift that is attributed to concern about future jobs and a preference for the economic policies of Mr. Reagan over those of Jimmy Carter, the only other President they remember. Many ethnic voters, especially Roman Catholics, are also moving into the GOP ranks as they become part of the more affluent middle class.

Yet, experts note, on many major issues the American public continues to hold Democratic positions. This includes support for increased social spending, the Equal Rights Amendment, and a slower military buildup, as well as opposition to a ban on abortion and to a loosening of pollution controls.

This seems to belie the President's contention that the expected crossover of Democrats reflects a ideological change. ''This is no mere political cycle, nor has it anything to do with the personalities of the candidates,'' he told campaign workers at the White House this week. ''We're attracting the support of people who have never voted with us before - not because they're deserting the Democratic Party, but because the Democratic Party has deserted them.''

On his final campaign swing Reagan first touches base in Boston to help out Senate candidate Ray Shamie. He then travels to New York, Michigan, Ohio, Illinois, Arkansas, Iowa, Wisconsin, and Missouri, before ending up in California for three campaign stops on Monday. His goal is threefold: to reinforce his lead in the major states that Mondale seeks to win; to help out selected House and Senate races; and to get out the vote.

Reagan will end his campaign on a positive note, says Mr. Black, stressing the theme of unity.

The GOP is pulling out all stops to pick up the 25 to 30 seats needed to regain working control of the House. Without such control, Reagan would face even more difficulty in a second term.