Democrats during the past half-century have won eight of 12 presidential elections and controlled Congress all but six years. They have been the governing party of the United States.

But they enter their national convention this week divided to the point of disintegration. Neither Jimmy Carter, nor Edward Kennedy nor anybody else in sight can satisfy the party and govern the country.

The famous Roosevelt coalition assembled the social groups that gave the Democrats their national majority. The South, which in those days meant the white South, was a key element. The working class of the Northern cities, largely ethnic and Catholic, furnished a second big component. Then came a middle-class constituency of business people, farmers and white-collar workers, strained in economic circumstances by the Great Depression and scattered throughout the land.

Blacks were first pulled into the coalition by New Deal economists, and then pushed by the civil rights movement. In the northern cities and in the states of the Deep South, they now constitute the most solid block of Democratic voters -- some 20 percent of the party's normal take in a presidential election. Their weight has been even further enhanced by a change in the values of the middle-class constituency.

Middle-class Democrats, having acquired a silk stocking outlook in the 1950s, developed in the next decade a disdain for mere material acquisition. They became possessed by a concern for higher things -- a more just society, a more noble foreign policy, a better balance of the ecology. Zeal for these causes made them the activist-militants of the Democratic Party.

The rise of the blacks and the silk stocking Democrats inevitably jarred the charter members of the old coalition. Southern whites found themselves harassed within the party on issues of race and national defense. The white workers of the northern cities came under fire because of their continuing zest for economic gain and their support for traditional values of state, church and family that legitimized material success. This process of decomposition has caused trouble for the Democrats ever since Lyndon Johnson was elected in the anti-Goldwater landslide of 1964.

He was forced out four years later by pressure from the middle-class liberals. Hubert Humphrey missed by just enough in the South and with northern workers and middle-class voters, to make Nixon the president. George McGovern, nominated by blacks and silk stocking Democrats, lost both the South and the white workers in the Nixon landslide in 1972.

Jimmy Carter put it all back together in 1976. The integrating force was an unusual, almost weird personality distinguished by a lack of sensitivity to internal contradiction. But the very qualities that brought opposites to his side as a candiate caused him as president to follow the zigzag patterns of policy that have caused so many voters to see Carter as weak and inept.

But it is not as though any other Democrat has shown more capacity to accommodate the party's rival factions. Sen. Edward Kennedy is nowhere in the South, and his working-class support stops at the Alleghenies. Sen. Henry Jackson is anathema to the silk stocking Democrats on defense issues. Secretary of State Edmund Muskie isn't militant enough for them or the blacks.

So Jimmy Carter enters the convention as the overwhelming favorite. Only an act of withdrawal by Kennedy, and a surge to a third man can deny Carter at this stage. But winning the nomination is only the beginning of new trouble.

The affair of brother Billy, bristling with unanswered questions, still hangs over the president. The white workers of the North scorn his weakness and abhor the social policies pushed upon them by blacks and silk stocking liberals. The same considerations have eroded pride in the native son felt by many Southerners. John Anderson beckons to the silk stocking Democrats. Maybe Carter can pull it out.

But to mobilize this potential constituency again, Carter needs outside help -- maybe from the course of events; more important from Ronald Reagan and his reaction to events. For while the Democrats cannot win the election, the Republicans can lose it. The decomposition of the Democratic Party does not automatically mean the emergence of a Republican majority. The sad fact is that the country is between majorities, and does not have, in either the Democrats or the Republicans, a true governing party. n