Public opinion experts are studying striking new findings that show sharp gains in the number of Americans who consider themselves Republicans and decreases in the number who consider themselves Democrats.

The shift appeared suddenly, in January, and has persisted since. It has been spotted in the new Washington Post-ABC News poll, in the Gallup Poll, in polls taken for President Reagan and in other polls, as well.

Whether it simply reflects a temporary national honeymoon sentiment for Reagan or a more permanent realignment is a question pollsters cannot answer. But if it is the latter, then its importance for the future is obvious.

At a minimum, if the trend continues, it will bring Republicans back to the position they held in the 1960s and early '70s, before the Watergate scandal.

Traditionally, the Democrats nationwide have held a huge margin over the Republicans as the party that people identify with. The new findings, while varying slightly from poll to poll, show that margin being cut in half. They also tend to show an increase in the proportion of Americans who consider themselves independents.

According to the head of the Gallup Poll, Andrew Kohut, the shift being found is much sharper than those than generally occur when a new president takes office.

"There have been slight aberrations in the past," Kohut said, but not like those being noticed now.

In the Post-ABC poll, conducted Feb. 19 and 20, 25 percent of those interviewed said they considered themselves Republicans, 32 percent said they thought of themselves as Democrats and 41 percent said they were independents. The remaining 2 percent declined to list any party sentiment. Those results give the Democrats a margin of 7 percentage points over the Republicans nationwide.

In polls conducted by The Post last year, the Democrats' margin was always double that or more.

Other polls show a similar change, although the exact percentage figures vary. Richard Wirhlin, Reagan's pollster, said his polls showed Republicans at 25 percent, Democrats at 34 percent and independents at 39 percent. In an interview last week, Wirthlin said the change has occurred during the past 40 days.

"I couldn't believe the first read," Wirthlin said, noting than in polling during the 1980 presidential campaign and immediately after he had seen no shift in party affiliation. Wirthlin said his polls last year constantly showed the Democrats holding more than a 20-point margin over the Republicans nationwide.

While some observers have said that the Reagan Landslide and sweeping GOP gains in the Senate marked 1980 as a "realignment" election, Wirthlin noted that he has not regarded it in that light -- until now.

The Republican pollster said his new findings are based on three nationwide polls since January totaling more than 6,000 interviews -- a number large enough to assert with conviction that the change has occurred.

At the Gallup Poll in Princeton, Kohut said that he noticed the change at the same time. Kohut said the Gallup organization conducts telephone polls each week with samples of about 500 people. The differences Kohut has uncovered are at least as striking as Wirthlin's.

At the time of the election, for example, Kohut said his telephone polls showed the Democrats with a 12-point edge, and in early December they held a 15-point edge over the Republicans in self-identified party affiliation. In his recent polls, that margin has varied from a low of 2 points to a high of 6 points, Kohut said.

At the same time, however, Kohut noted a peculiar variation: While Gallup's telephone polls consistently have shown the Democrats losing support and the Republicans gaining, the firm's in-person interviews have not. s

"i don't know what to make of it," Kohut admitted. Ordinarily, he noted telephone interviews result in a greater number of people labeling themselves as independents than do in-person interviews, but show no overall difference in the proportions considering themselves Democrats or Republicans.