President Reagan's reelection victory yesterday was a broadly based triumph delivered by the middle classes, whose members concluded it is the Republican Party that can best maintain prosperity and deal with the nation's other problems.

This instant portrait of the result was drawn from interviews with more than 18,000 voters conducted yesterday for ABC News.

The interviews confirmed the theory that pocketbook issues were paramount and that such widely publicized matters as religion in politics, the Equal Rights Amendment, the nuclear freeze and environmental concerns played no major role in the outcome.

There was also evidence in yesterday's exit polls that Geraldine A. Ferraro's selection as the Democratic vice-presidential candidate may have cost her party votes.

NBC, for example, reported that 16 percent of the voters said her presence on the ticket encouraged them to vote Democratic, but 26 percent said her nomination made them less likely to support her party. Intimation of Historic Shift

One of the most significant aspects of the evening was its intimation of a historic shift within the electorate toward the Republican Party, which for now is perceived as the political institution best suited to govern.

This feeling, the exit polls revealed, has infected nearly one in five Democrats, nearly two-thirds of the independent voters, more than 60 percent of the newest voters -- aged 18 to 24 -- and an equal percentage of the "Yuppies" of the Baby Boom generation.

It found tangible expression in a major increase in the total vote for Republican congressional candidates as well as in Reagan's monumental victory.

In the wide mosaic of this election, Democrat Walter F. Mondale appealed strongly to only one segment of American society -- the underclass and those concerned with its plight.

He got 70 percent of the vote of people with incomes of less than $5,000 a year and a little over 50 percent of the vote of those with incomes between $5,000 and $10,000.

In all other income groups, the exit polls showed Reagan triumphant with margins as great as 70 to 30 percent among the most affluent voters.

There was also a clear racial split of the electorate. White voters went for Reagan by a ratio of nearly 2 to 1; roughly nine out of 10 black voters chose Mondale.

There were some notable exceptions to these broad tendencies. For example, seven out of 10 Jewish voters -- a generally high-income religious denomination -- favored Mondale over Reagan. On the other hand, Reagan ran most strongly in some of the poorest states in the union, including the Deep South.

Specific policy issues in the 1984 campaign were overshadowed by the personalities of the candidates, according to The Washington Post's polling director, Barry Sussman.

Reagan voters saw him as a strong leader who would keep the country prosperous and maintain its military strength. Mondale voters saw their candidate as an advocate of fairness and social justice, admired his performance in the debates and approved of his positions on the federal deficit and the nuclear freeze. Support by Union Households

There are many measures of the breadth of Reagan's appeal. He won by wide margins among every age group and in every region of the country. Union households gave him 47 percent of their votes, despite the strong endorsement of Mondale by the AFL-CIO.

The best-educated Americans, represented by voters with postgraduate degrees, favored Reagan narrowly -- 52 to 48 percent. Among college graduates with bachelor degrees, Reagan's majority was more than 2 to 1.

The exit polls yesterday revealed another shift of significance to both Reagan and the Republican Party. Virtually every identifiable ethnic group in America gave its votes to Reagan -- Poles, Slavs, Italians, Germans, WASPs, Scandinavians and Irish.

The exceptions were Orientals, who preferred Mondale by 54 to 46 percent, and black voters, who gave Reagan only 10 to 12 percent support.

In addition, Hispanic voters nationally appeared to have given Mondale a modest majority. But in Texas, exit polls show Mondale getting 72 percent of the Hispanic vote.

Among the major religious groupings, Reagan carried the Protestant vote by 2 to 1 and the Roman Catholic vote by roughly 3 to 2.

Remaining from all this are profound questions about the long-term drift of American politics. Specifically, has the United States just engaged in a "realigning" election that may insure Republican control of the White House for years to come and perhaps for the remainder of this century? There are those who believe that it does, including Gordon Black, the polling director for the newspaper USA Today, and Horace Busby, an independent business and political consultant whose prescient forecast of Reagan's electoral landslide in 1980 has given him cachet as a formidable analyst and seer.

Black, just before the election, declared that Reagan would achieve "one of the great, historic landslides of all time" in "the most important and fundamental election since 1936 . . . . This is a realigning election in which Reagan appears to be capturing the young in a manner similar to what Franklin D. Roosevelt accomplished in 1936."

Busby is another exponent of that view. "At this election," he wrote on Nov. 1, "the demographics of age will be supremely important. Will the 104 million young adults, 18 to 44, take over the electorate, displacing the 70 million adults over age 45? Will they vote . . . predominantly Republican? If so, that is ominous for the Democrats because once Americans begin voting, they continue for the same party . . . .The 1984 election could be the Republican equivalent of 1936 which created for the next 16 years a Democratic presidential majority . It will ratify 1980's conservative turn. If the margins are wide, the effect may be to entrench the GOP through the next three elections. The stake is not 'Four More Years.' It may be, for both parties, the rest of the century."

Richard B. Wirthlin, the principal poll-taker for the White House, cautiously echoed those words on Monday: "One of the things this election can show is a change of partisanship in the United States. If you want to call it realignment, fine."

One definition of a "realigning election" is the creation of a new majority party that dominates not only presidential elections but congressional, state and local elections as well, a result the Democrats achieved in the 1930s after decades of Republican predominance.

Mathematically, no such sweep can occur this year, although voter identification with the Republican Party is at its highest level in a half-century.

Some preelection studies found an equal number of voters identifying with the two major parties, a substantial shift from the past when Democrats outnumbered Republicans by 15 percentage points or more. Pollster Black attaches great significance to the strong Republican showing.

Busby is less sure. His theory is that the American public rejects the concept of a single party dominant at all levels. It consciously wants a division of power between Republicans and Democrats to fulfill the "checks and balances" philosophy of the Constitution.

"On poll after poll," he wrote, "Americans say it is desirable to have Mr. Reagan in the White House, precisely for the purpose of serving as a check on the Democrats. Conversely, a recent Louis Harris poll found 60 percent favor Democrats to control the House as a check against Mr. Reagan. The conceit of Washington is that the people are eager for government to act; in fact, since the 1790s, Americans have feared activist government, they want to keep it bridled."

So far as a "majority party" is concerned, the Democrats continue to fill that role. They control, by Busby's calculations, 75 percent of all elective offices in the country. Yesterday's results, while showing some Republican gains in Congress and in statehouses, have not changed that basic calculus.

At the beginning of this long election process, the Mondale wing of the Democratic Party staked its hopes on the mathematical potential of a revived Democratic "coalition" whose principal components would be American labor unions, women, blacks, Hispanics and lower-income groups. That calculus was flawed. Reagan drew significant support from all these groups with the exception of black voters.

There were two major miscalculations in the Democratic strategy of coalition. The first involved the "gender gap," the idea that women voters -- especially with a woman on the ticket as vice-presidential candidate -- would produce a decisive majority for Mondale.

Not only did Reagan win a majority of the women voters, but he benefited hugely from a male "gender gap," getting almost twice as many votes from men as Mondale.

The exit polls produced strong evidence that "feminist issues" carried far less weight than many Democrats had assumed. Only 6 percent of all voters and 11 percent of women voters, for example, described themselves as "strong feminists" who favored the Equal Rights Amendment. These women gave Mondale 77 percent of their votes. But the vast majority of women -- 89 percent -- divided their votes 60 to 40 in favor of Reagan.

A second miscalculation involved the Democratic emphasis on the registration of black voters as the supposed key to victory in the South.

These registration efforts were matched or exceeded among white voters with the result that Reagan swept the South and, nationally, received nearly two-thirds of the white vote. In Mississippi, 85 percent of the white vote went to Reagan.

Throughout the South, Reagan's share of the white vote approximated 75 percent and was even higher among white males. At the same time, black voters were going 9 to 1 for Mondale. This startling gulf between white and black voting patterns caused speculation about a new period of racial polarization in American politics, especially in some of the southern states.

The longer-term questions about party strength in America involve younger voters. Reagan this year found his strongest support among the young and near-young, those aged 18 to 44. They include the enormous Baby Boom generation born in the 15 years after World War II.

Yesterday they chose Reagan and Republicanism. But it is too early, in the opinion of Busby and some other political analysts, to draw sweeping conclusions from that fact.

If, as the exit polls suggest, the state of the economy was a major factor in the appeal of Reagan and the Republican Party to the young, then an economic downturn over the next four years could drive those voters to the Democrats as occurred in 1958, just two years after the massive presidential victory of Dwight D. Eisenhower.

Only future elections will determine if the alliance between the young and the Republican Party will have the longevity Roosevelt achieved with his New Deal coalition a half-century ago. That coalition has now been shattered by the influences of demographic, economic and sociological change; a new political dawn is here.

As of today, the Republican Party is the party of the vast American middle classes made up of blue- and white-collar workers, entrepreneurs, Catholics and Protestant fundamentalists, Yuppies and new voters. To make of them a lasting coalition remains a formidable task.