American politics -- dominated for 50 years by a Democratic coalition built around urban workers, minorities and the rural poor -- has been turned on its head in the last two presidential elections and is now dominated by a Republican coalition built around the rich and the upper- middle class.

If this Reagan revolution can be converted into an enduring partisan realignment, the ascendancy in the 1930s of those dispossessed by the Great Depression will have been replaced in the 1980s by the ascendancy of the affluent.

The energies of such a coalition would be spent overwhelmingly on continuing the pattern of tax, spending and regulatory decisions of the Reagan years in which it is the affluent who benefit most, whose taxes are reduced most, whose incomes rise fastest during economic recovery and whose acquisition of wealth is facilitated by actions of the federal government.

The growing strength of the Republicans -- now nearly equal to that of the Democrats in terms of how voters identify themselves -- has significance beyond realignment of the parties. Just as the Democrats' once-solid partisan advantage acted as a brake on conservative policies under such presidents as Eisenhower and Nixon, so today's revived GOP not only supports the current administration's policies redistributing income upwards but will act as a firm brake on traditional Democratic initiatives if that party regains control of the Senate and the White House. In the current session of Congress, the struggle to shape legislation overhauling the nation's tax system will be a major test of the balance of power in the electorate.

In strictly political terms, the most striking element of the Reagan coalition is its restoration of political divisions along class and income lines reminiscent of those of the Great Depression. From the early 1950s through the mid-1970s, class divisions between the two parties were muted. But recent years have again seen a strong correlation between income on one side and voting and partisan commitment on the other.

The extraordinary intensification of class voting is clear when the landslide victories of two Republican presidents running for re-election are compared: Eisenhower in 1956, when he won with 58 percent of the vote, and Ronald Reagan in 1984, when he won with an almost identical 59 percent. But despite their similar overall results, the two mandates had strikingly different compositions.

This point is illustrated in the following chart based on data compiled by Martin P. Wattenberg of the Univeristy of California at Irvine from National Election Studies (NES). The voting population is divided into five income groups: the poor (lowest 10 percent), lower middle class (11th-30th percentile), middle class (31st-60th percentile), upper middle class (61st-90th percentile, and rich (top 10 percent). chart

Eisenhower won in every income group, and all groups except the rich gave him almost the same degree of support -- 56 to 59 percent of their vote. In statistical terms, there was no difference between his support by the poor and his support by the upper middle class.

Reagan, in contrast, was carried to victory by the nation's haves and was decisively rejected by the have- nots. In the 1984 election, the difference between Reagan's support by the poor and by the upper-middle class was 32 points. The poor and lower-middle class, which had given Ike 59 and 56 percent respectively, gave Reagan 32 and 43 percent.

The re-emergence of class voting patterns has been paralleled by growing class divisions in partisan allegiance. This can be seen in the following NES-Watteberg chart showing, by income group, Democratic allegiance as measured by the number of voters per 100 who call themselves Democrats minus the percenatge who call themselves Republican. chart

In 1956, Democratic strength varied only slightly according to income group -- except for the rich, who then and now were firmly in the hands of the GOP. Though the Democrats were somewhat stronger toward the lower end of the scale, the poorest voters were only five points more Democratic than the upper middle class.

The same pattern held true in 1960. But by 1984, the pattern of partisan commitment was following clear income/class lines: the poorer the voter, the more likely he or she would be a Democrat. The five-point spread between the poor and upper- middle class had grown by 1984 to a 36-point difference, and the 40 point spread between richest and poorest had grown to 69 points.

Reagan, then, was elected by a constituency of the affluent, and his political revolution has helped shape a new majority of the economically privileged.

This same period, according to Census Bureau data, has seen a shift in the distribution of income. From 1980 through 1984, the median income for all families dropped slightly, from $26,500 to $26,433 (in constant 1984 dollars). But when median family income is calculated by income group -- the bottom 40 percent, the top 40 percent and the top 10 percent -- a clear pattern of redistribution emerges: chart

Changes in the pattern of income distribution result from a host of forces, including recession, recovery and government decisions on taxes and spending. In 1984, the Congressional Budget Office estimated that tax and spending legislation enacted since Reagan took office -- particularly in 1981 -- would have the following net effects for 1983 through 1985 for different income groups: chart

In the classic American political tradition of "dancing with the guy who brung you," the Reagan administration has rewarded its new, affluent constituency of Republicans. Just as the Democratic majority of the 1930s was rewarded by a Democratic administration with Social Security, the National Labor Relations Board, unemployment compensation and tax policies designed to redistribute income downwards, the Reagan administation has won enactment of tax and spending legislation redistributing income upwards, has weakened the federal regulatory aparatus governing the relationship between corporations, workers and consumers, and has lessened the tax burden on wealth through reductions of the capital gains and estate taxes.

While Reagan's success has been widely credited to his popularity and broad array of telegenic skills, he could not have achieved this substantial alteration of the federal government without a major change in the electorate itself. In this context, the most important changes have been not only the revival of class clevage between the parties but the simultaneous growth in the Republican Party's competitive strength.

Market Opinion Reseach (MOR), a Detroit-based polling firm, has found that from 1952 to 1978 the Democrats enjoyed a consistent and overwhelming advantage over the GOP in terms of the way voters described themselves. During those years (and probably as far back as 1932 if data were available), the percentage of persons identifying themselves as Democrats was 14 to 30 points higher than those calling themselves Republican.

Despite the Democratic dominance in those years, Republican presidential candidates Eisenhower and Nixon won decisive victories and Eisenhower even carried both branches of Congress in 1952. But unlike the Reagan victories of the 1980s, the triumphs of the 1950s, 1960s and 1970s saw no conservative revolution in public policy accompanying Republican control of the White House. One central force preventing such a revolution was the overwhelmingly strong Democratic majority in the electorate, which acted as a brake on major initiatives from the economic and ideological right. A substantial proportion of voters remained committed to the principles -- if not the presidential candidates -- of a party supporting a strong, active federal government willing to intervene in behalf of those out of work and those organizing the workplace.

This Democratic restraint on a conservative political agenda began to erode between 1978 and 1980 when the Democratic advantage reflected in how voters described themselves fell from 23 points (53 Democratic, 30 Republican) to 13 points (53-40), according to MOR figures. The shift has had consequences far more important than the momentary status of a political party.

By 1984, a Republican Party increasingly dominated by the upper- middle class and the rich had achieved near parity with the Democratic Party, running just three to five points behind in most opinion polls of party idenfitication. Even this slight disparity disappears when turnout rates, which are much higher for the well-to-do than for the poor and lower-middle class, are taken into account.

The strengthened GOP's partisan gains mean that the electorate has applied a conservative brake on legislative policy. Just as the Democratic majority in the electorate limited the scope of policies permitted to GOP administrations in the 1950s, 1960s and early 1970s, the GOP's current parity has created a conservative veto on liberal, Democratic initiatives in the 1980s. This conservative brake has severely undermined the Democratic Party's groping efforts to redefine its goals after the defeats of 1980 and 1984. A Democratic Party that set the national agenda for nearly 50 years has been consistently on the defensive since Reagan took office.

The Democrats' inability to take the initiative was reflected most recently and perhaps most graphically in the passage by Congress of the Gramm-Rudman-Hollings proposal to mandate severe budget cuts in order to achieve a balanced budget by 1991. House Democrats voted xxx-xxx on the measure. Senate Democrats split 22-22, and among the supporters was Sen. Edward M. Kennedy (D-Mass.), for years the Democrats' liberal standard bearer. Kennedy epitomizes the quandry Democrats now face, given the changed electorate: "In order for (Kennedy) to establish credibility as a liberal Democrat with the general public, he first has to gain fiscal credibility," a key Kennedy adviser said.

In a development even more threatening to the Democratic Party, a series of focus groups financed by the Democratic National Committee has found that the issue considered most advantageous to the party -- "fairness" -- appears to have turned into a potential liability among major portions of an increasingly conservative and self- interested electorate. "Fairness," according to those who conducted the studies, has come increasingly to be seen as a code word for "giveaways" to the poor.

What all this suggests is that even if the Democratic Party regains control of the Senate and White House, the scope of its legislative program will be severely restricted by conservative forces in the electorate. In this light, it is quite possible that the Republican Party and its conservative base could lose political control of the federal government in 1988 but retain de facto control over the national debate and the legislative agenda.