In Mississippi, the Democratic Party nominated a black state legislator as its candidate for Congress in a Delta district where nearly half the electorate is black.

In Alabama, Democrats are looking to blacks to help oust a first-term Republican congressman in the steel-making suburbs outside Birmingham. There are at least four similar races where Democrats are counting on blacks across the South.

In the Northeast and Midwest, the theory not too many months ago was that population loss and reapportionment would hurt the blacks in Congress, perhaps forcing some to run against each other. Now it seems likely that blacks will gain seats in the next Congress, almost certainly one and perhaps three or four. The black vote is one of the great question marks in the Nov. 2 elections. It is not so much which way blacks will vote -- most are expected to vote Democratic -- as it is how many will turn out to cast their ballots.

The voting rate for blacks has tended to be about 10 percentage points lower than that of whites, according to the Joint Center for Political Studies, a Washington-based research organization. But that rate is subject to wide fluctuations; it increases substantially when a black candidate is running or there is a white candidate who is perceived to be anti-black.

The question now is whether black opposition to President Reagan will rub off on GOP candidates or lead blacks to stay home.

Republican strategists are hoping that Reagan's recent overtures to blacks will at least take some of the edge off the opposition toward him and keep black turnout at its normal low level for off-year elections. Democrats believe that heavy black turnouts can deliver large gains for them.

"What we would like is just no greater turnout," said one GOP strategist.

Blacks comprise 20 percent or more of the population in 86 of the 435 U.S. House districts, according to an analysis by the joint center. More than two-thirds of those districts are in the South, where Democratic strategists hope there will be large enough turnouts to knock off several freshman Republicans.

They include Albert Lee Smith Jr. of Alabama, Eugene Johnston of North Carolina, Thomas Hartnett and John L. Napier of South Carolina and Thomas J. Bliley Jr. of Virginia, who were elected on Reagan's coattails in 1980.

Tacticians for two Democrats, former Massachusetts governor Michael Dukakis and New York Lt. Gov. Mario Cuomo, say they believe that their emphasis on jobs and unemployment and ability to tie their opponents to Reagan were responsible for the substantial black turnouts in Roxbury and Harlem that were critical in their primary election victories.

Two years ago blacks were in despair over their political clout. Although the rate of black voter turnout was slightly higher than normal and the ballots they cast were overwhelmingly for President Carter, they had failed to make a difference in the general election.

And, a half-dozen of the black incumbents in the House had to worry that their seats would be among the 17 in the Northeast and Midwest given to Sun Belt states in reapportionment after the 1980 census.

But the incumbents, aided by blacks in the state legislatures, were able to make deals with Republicans and use the courts to challenge congressional redistricting plans that threatened them.

Now, blacks are certain to pick up one new seat in Congress, the heavily black and Hispanic Brooklyn district formerly represented by Democrat Frederick W. Richmond, who resigned from the House last month after pleading guilty to criminal charges.

A strong race also is being mounted in Kansas City by Democratic Missouri state Rep. Alan Wheat.

In Mississippi, state Rep. Robert Clark is the Democratic nominee in a newly carved Delta district where he must win a substantial number of white votes to be elected..

The Republican National Committee has declared a goal of electing at least one black member to Congress this November and has contributed $28,000--the maximum permitted by law--to three candidates: three-term Dallas City Council member Lucy Patterson, who is attempting to unseat Rep. Martin Frost (D-Tex.); Pennsylvania state Sen. Milton Street, who is attempting to upset Rep. William H. Gray (D-Pa.) in Philadelphia, and bus driver Shirley Gissendanner, competing for a new district in San Diego. All are said to be in uphill battles.