Why is Jimmy Carter as good as amassing Democratic votes and so poor at being president? The answer lies in the crazy, mixed-up amalgam of conflicting groups and opposing values that make up the party.

Carter, and only Carter, possesses the singular, even weird, qualities required to hold the mix together. But those very qualities yield the hesitant, moralistic, personalized brand of government that distinguishes the Carter administration.

Southern whites, his own kind, are the starting point of Carter's strength inside the party. Texas provided the votes that put him over the top in the balloting at the convention, and his most solid support came from below the Mason-Dixon line. A recent study of 40 different kinds of national regions, undertaken by the polling firm of Targeting Systems, Inc., found that one of the areas of the country most favorable to Carter lay in the "piney woods" districts of the rural South.

Blacks constitute another element in the Carter coalition. In the analysis of different districts made by Targeting Systems, Carter did brilliantly in both urban renewal areas, where the poorest blacks live, and in areas heavily populated by the emerging black middle class.

A third group with whom Carter does at least passably well is formed by the unionized workers of the Northern cities. Sol Chaikin of the Garment Workers gave one of the secondings speeches on behalf of the president. According to the Targeting Systems survey, Carter runs very well in districts designated as "Middle American Blues" -- "inner suburbs" where people are "blue collar, high-school educated . . . family oriented . . . white and native born." Finally, there is that section of the middle class that has experienced the "greening of America" -- passed beyond material considerations to a quest for higher things such as a better environment and more self-expression. According to the Targeting Systems survey, Carter does exceptionally well in districts identified as "Bohemia" -- urban areas dominated by "white collar, upper middle-class persons" who rent apartments and are "not currently married."

The merest glance at those categories shows how much they are at odds. Piney woods Southerners and urban blacks stand poles apart when it comes to such issues as the welfare state and government spending. Blue-collar workers and swinging singles are at daggers drawn in their attitudes toward family, church and state.

No other Democrat of national prominence can begin to corral all of these opposites. Carter gets them by being himself a strange blend -- Southern, religious and rural in personality; extremely liberal on civil rights; generally favorable to economic underdogs; and disposed to leave abortion and other social issues to the individual conscience.

Enthusiastic support from all groups, to be sure, does not go to Carter. But Carter -- and only Carter -- is acceptable to the full range of the Democratic constituency. He is "the remainder candidate," as the pollster Peter Hart once put it. He assembles bits and pieces of everybody else's base. But those bits and pieces give him a commanding majority inside the Democratic Party.

Inevitably, however, the qualities that enable Carter to dominate the party find expression in his performance as president. Being a mix of many things himself, he is not naturally decisive. His instinct is to go for all possible good goals without being sensitive to their incompatibility. Thus he stimulates more jobs without gauging the impact on inflation. He next goes after inflation without sensing its effect on unemployment.

That innate tendency finds reinforcement in the Carter constituency. The president's Southern backers revolt against his liberal policies. The blacks and the middle-class liberals grow restive when he turns conservative. So his supporters ran home his personal inclination to zig and zag.

All of this came together in the windup of the Democratic convention. Carter had won the nomination by a landslide, navigated the rapids of the platform skillfully and achieved about as much unity as he could have expected. The delegates wanted to be wowed in the acceptance speech. But Carter spoke as president.

He was defensive about his record, and received some jeers. His effort to wind himself in the mantle of past Democratic heroes backfired in the gaffe that caused him to speak of a former vice president as Hubert Horatio Hornblower. His rhetoric paled in contrast to the magnificent speech delivered earlier by Sen. Kennedy; the contrast was underlined by Kennedy's coolish demeanor when he finally appeared on the platform with the president.

So the country now enters a campaign with both parties in far-out positions. Carter's record invites Ronald Reagan to hammer away at "liberal follies." Reagan's past demands that Carter hit what he calls the Republican "fantasy world."

Whether those sharp attacks will have any impact on the great mass of uncommitted voters at the center is a baffling question. Perhaps not. In which case, the outcome of the election, like so many other things this year, will be up to the play of events.