An article yesterday incorrectly stated that 4 million of 12 million newly registered U.S. voters turned out in the November election. The study did not estimate how many newly registered voters turned out. The report said 4 million of 12 million newly registered voters were registered by group efforts.

A big increase in voter registration in 1984 helped erase a two-decade decline in voter turnout, and the Republican Party was the early beneficiary of the new participation, a study group reported yesterday.

The findings were released as President Reagan and Vice President Bush completed the final step in the electoral process, with a joint session of Congress formally ratifying their November landslide.

The Committee for the Study of the American Electorate (CSAE) said that 53.3 percent of those eligible voted last November, an increase of 0.7 percentage points over 1980 and the first time that participation has risen since it peaked at 62.8 percent in 1960.

The turnaround was directly attributable to the increase in registered voters. Between 1980 and 1984, the number of registered voters rose by 12 million, bringing to 73 percent the percentage of eligible Americans who were registered to vote. That was the first percentage increase in registered voters since 1964.

However, only about 4 million of the 12 million new voters turned out in November, which meant that the percentage of registered voters who took part in the election continued to decline. The CSAE study said that turnout among registered voters is now 10.8 percentage points below 1960 levels.

A companion survey of 883 newly registered voters, taken Nov. 30-Dec. 6, found strong support for Reagan and the Republican Party.

The newly registered voters -- those who signed up in 1983 or 1984 -- said they voted for Reagan by 61 percent to 32 percent. More significantly, they said they had voted Republican in congressional races by 45 percent to 39 percent.

Democrats last year launched a major effort to register women, minorities and poor people, hoping that an expanded electorate would help them recapture the White House. But Peter D. Hart, who conducted the poll for CSAE and was Democratic candidate Walter F. Mondale's pollster during the campaign, found that hope turned out not to be the case.

"The Democratic plan to expand the electorate and think that everything will work out fine just will not fly," Hart said.

The one group that strongly supported Mondale was newly registered black voters, who went Democratic by 83 percent to 12 percent. Hart said that Jesse L. Jackson's presidential campaign and registration drives aimed at black voters had a measurable impact last year.

Hart's poll found that a disproportionate number of the new voters were from the South, that 57 percent of them nationally were between the ages of 18 and 24, that 28 percent were fundamentalist Protestants and another 31 percent were Roman Catholics.

Nearly a majority of the newly registered voters indicated an allegiance to the Republican Party. Hart found that 49 percent considered themselves Republicans or independents with Republican leanings; 38 percent said they were Democrats or had Democratic leanings; 13 percent said they were independents.

In the South, 55 percent of new voters identified themselves as Republicans. Curtis B. Gans, CSAE director, said that finding indicated that "the age of Democratic dominance in the South is dead."

Hart's survey also showed strong Republican allegiance (53 percent) among new voters in the East. In the Midwest, 44 percent said they were Republican to 43 percent who called themselves Democrats. In the West, the Republican margin among new voters was 46 percent to 37 percent.

"The key for the Democratic Party . . . is not to reestablish the New Deal coalition," Hart said, "but to deal with new voters coming into the electorate."

The survey found that 68 percent of newly registered voters ages 18-20 supported Reagan. But Hart said many of the new voters have weak ties to either party. "The Democratic Party has to give a reason to follow them. But they're not lost forever," he said.

The joint session of Congress that ratified the electoral college vote of 525 for Reagan and Bush and 13 for Mondale and running mate Geraldine A. Ferraro lasted less than half an hour yesterday.