HERBERT HOOVER was the last Republican presidential candidate black voters supported. Ever since, Democrats have kept the black vote in their hip pocket while they wooed other voters. But Democrats ought to recheck that pocket -- its contents have been changing.

About a third of blacks now eligible to vote are too young to have been part of the civil-rights movement. To them, the facts that Roosevelt brought government help to blacks along with the rest of the downtrodden, that the Kennedys gave a blessing to the civil-rights struggle and that Johnson gave it some political teeth are pieces of history, not articles of faith that dictate black allegiance to the Democratic Party. The young blacks one expects to register and vote will do so not because they are blindly loyal to a party but because they have personal interests they wish to protect and personal concerns they want addressed. They are going to vote for the candidate who responds to their need, regardless of that candidate's party.

This tendency toward party-independent thinking is shown in a poll conducted late last year by Gallup and the Joint Center for Political Studies. In prior polls, blacks differed markedly from whites in their primary concerns. Those earlier concerns were global; they dealt with blacks as a group. And they fit well within broad, old-line Democratic goals. This time, for the first time, blacks and whites voiced the same top concerns. Seven of the top 10, in fact, were held in common. Black and white voters are most concerned about their jobs, their income and its ability to provide for their needs, and the corrosive effects drug abuse and crime are having on their communities. These are the concerns Democrats must address this time around if they want votes from a black votership that is increasingly party-independent and oriented toward personal issues.

The erosion of unquestioning Democratic Party loyalty also shows up in a softening in party identification. Nearly eight out of 10 blacks still describe themselves as Democrats. But, fewer than half of Democratic blacks -- and only 37 percent of those under 30 -- said they were "strongly" Democratic. Most black Republicans used to be over 50 -- the only blacks old enough to remember what Republicans had done for them in the pre-Roosevelt era. Now more than half of blacks who say they are Republican are under 30.

So the younger segment of the black votership, nearly a third of all black voters, contains most of the Republicans, and of those who call themselves Democrats, more than six out of 10 are not staunch loyalists. But even among older blacks,Democratic ties are weakening. This is the stick that should force the attention of the Democratic Party.

Here is the carrot. Consider what the black voter has to offer a responsive Democratic Party willing to negotiate to build a coalition. Start with this: One-fourth of the Democratic vote is black. The fine print is even more gripping than that. The geographic concentrations of black voters are such that as much as 30 percent of the vote on Super Tuesday -- when several southern states hold primaries -- is likely to come from blacks. Only one Democratic candidate, the Rev. Jesse Jackson, has any reason to take these votes for granted.

Looking to the general election, a unified black vote will make the difference between whether the Democrat or the Republican wins in the District of Columbia, Texas, Florida, Louisiana, Georgia, Alabama and North Carolina. If black voter registration and turnout were to increase by just 5 percent, the black vote would also be decisive in Arkansas, Massachusetts, Mississippi, South Carolina and Tennessee. And it would be a major factor in the outcome in Kentucky, New York and Virginia. Those states contain 170 of the 270 electoral college votes needed to win the presidency. Effective registration and get-out-the-vote campaigns in black communities could deliver the presidency to the Democrats.

Perhaps as important as what will happen if blacks do live up to their Democratic voting potential is what can happen if they do not. Blacks could feel locked out of the party, for example, if Jackson is again snubbed by the party or if the party will give nothing substantial in return for black votes. Let's say a fourth of the black voters demonstrated their displeasure by boycotting the election or voting for an independent. Those 2.6 million votes would not have changed the 1980 or 1984 outcomes; the winning margins were too overwhelming. But they would have given the presidency to Nixon instead of Kennedy in 1960, or to Ford rather than Carter in 1976. At this point, the 1988 contest looks much more like a Ford-Carter than a Mondale-Reagan race.

In considering whether to take the black vote for granted, the Democrats had also better look beyond the presidency. Blacks make up 20 percent or more of voters in 71 congressional districts. The increase in black voter turnout in 1986 won the Senate races for Democrats in Alabama, California, Georgia, North Carolina and Louisiana and played a crucial role in Florida and Maryland. Three of the five Republican seats that changed hands to give Democrats the majority could not have been won without the black vote. (See box).

At the state level where apportionment decisions are made, 46 new black state legislators won office in 1986 because of black voter turnout. In fact, one of the strongest predictors of whether a legislative district is or can become Democratic is the percentage of black voters in the district. The higher the percentage, the greater the likelihood.

If the Democrats truly want decisive power in the '90s, they have to elect enough Democrats to influence redistricting. The surest way to accomplish this is to not take the black vote for granted. Jesse Jackson has demonstrated that blacks vote when they feel they have a reason to do so. The Democrats can choose to provide a good reason and be richly rewarded, or they can assume the black vote is theirs, free of charge, and pay dearly.

Rep. Mervyn Dymally is chairman of the Congressional Black Caucus and former lieutenant governor of California.