The unspoken assumption among most of those involved in the effort to reshape the Democratic Party after two successive landslide presidential defeats is: We've got the blacks; let's concentrate on getting more whites.

They don't put it that crudely, of course. They prefer to talk about "getting back in the mainstream" or "erasing the special-interest label."

But there are few Democratic leaders who are unaware of the implications of the racial polarization in the Jimmy Carter and Walter F. Mondale votes. Both those men got more than nine out of 10 blacks to vote for them but gained the support of fewer than four out of 10 whites. As a result, they were wiped out in 1980 and 1984 by Ronald Reagan.

As they prepare for 1986 and 1988, the emphasis among the Democrats is clearly on improving their acceptability to middle-class white voters by demonstrating their concern about budget deficits, a strong, efficient defense, retirement benefits and tax burdens.

But while most Democrats are busy courting the straying white voters -- from yuppies to hard-pressed farmers -- there is growing intellectual and political dissent among blacks.

In the past few weeks, there has been substantial discussion among black politicians, and almost none among whites, about one striking finding from a survey of black voters taken before the 1984 election.

That survey showed that 59 percent of the 1,150 respondents would have supported Jesse L. Jackson if he had decided to run as an independent in November of 1984 against Mondale and Reagan. This astonishing response, if carried out, would have left Mondale with less than one-third of the popular vote. And it "makes a Republican a shoo-in in 1988, if Jackson decides to run as an independent next time," commented Rep. Mickey Leland (D-Tex.), the chairman of the Congressional Black Caucus and a 1984 Mondale supporter.

Jackson is, of course, saying nothing about his 1988 plans, and he is the key. Three-fourths of the blacks surveyed rejected the idea of a separate black party as an abstraction, while a heavy majority said they would follow Jackson as an independent presidential candidate.

The knowledge that perhaps 6 million of the 10 million black voters were ready to follow Jackson out of the Democratic Party and might be again certainly casts a different light on the intra-party policy debates. The key finding from the National Black Election Study was reported by Shirley Hatchett and Ronald Brown of the University of Michigan Institute for Social Research in a recent bulletin of the Congressional Black Caucus Foundation.

The lack of notice it drew from white Democratic Party leaders is an indication of the party's preoccupation with the pursuit of turned-off white voters. But it is also evidence to many blacks of a growing gulf between the party leadership and the Democrats' most loyal constituency.

Leland, who calls himself "very much of a Democratic Party insider and loyalist," says: "It's harder and harder to defend that position to people outside the party when the Democratic Party just does not show any leadership on black people's behalf."

The national survey indicated that Jackson's support was concentrated among younger and better-educated blacks, the ones who are sure to be the political activists of the next decade or more.

One of them, William Nelson Jr., of Ohio State University, told a panel on black politics at the recent American Political Science Association convention that "black leaders have been forced to reassess their relationship with the Democratic Party." As he put it, "the Democrats have decided that black leaders should be shunned. . . . They blame us for whites' rejecting the Democratic candidate."

While conceding that no organization base exists for a black third party, Nelson said that he expected to see many more black politicians pursue "a new policy of pragmatism . . . seeking goals that are not dependent on white support."

Agreeing, Thomas E. Cavanagh, a specialist in black politics at the National Research Council, said in a recent paper that more blacks will run as independents in local races in the South, in order to avoid the Democratic runoff primaries that have been a target of Jackson's complaints. Success for some of those campaigns would obviously add to the credibility of a Jackson independent candidacy in 1988.

As Cavanagh pointed out, the relative advantages and disadvantages for blacks' working entirely within the Democratic Party "will continue to divide black political elites along generational lines, and to pit black electoral and party leaders dependent on white constituent and financial support against indigenous community and religious leaders who played a key networking role in the Jackson campaign."

But the overlooked finding that 59 percent of the black voters would have followed Jackson out of the Democratic Party in 1984 cannot be ignored by Democratic leaders forever.