Robert G. Clark should be a shoo-in to defeat Rep. William W. Franklin (R-Miss.) in next month's election.

Clark, who 16 years ago became the first black to serve in the Mississippi Legislature since Reconstruction, is running in a tailor-made, majority-black district against a first-term white incumbent closely allied with the Reagan administration.

Franklin has opposed a number of federal programs to help the unemployed and voted against legislation designating the Rev. Martin Luther King Jr.'s birthday a national holiday -- hardly popular stands in a poverty-stricken Delta district where 53 percent of the voting-age population is black.

Scores of prominent black leaders -- including Jesse L. Jackson, Coretta Scott King and Detroit Mayor Coleman A. Young -- have targeted Clark's campaign as the best chance for a black candidate to pick up a seat in the House of Representatives this year.

Yet Clark isn't a shoo-in, and his uphill battle is indicative of the problems black congressional candidates have faced elsewhere in trying to capitalize on the surge of black political involvement and voter registration generated by Jackson's "Rainbow Coalition" presidential campaign.

Most political observers agree that Franklin may hold a slight edge in the Mississippi district by virtue of his incumbency. In 1982, before the latest redistricting gave blacks a clear majority, Franklin defeated Clark by fewer than 3,000 votes in a race for an open House seat.

"The key to it is how much the black community will transfer its loyalty to any candidate and translate that by going to the polls on Election Day," said Frank Carlton, a former state legislator from Greenville. "Mississippians understand the value of incumbency . . . . In the absence of something outrageous, we have a history of returning people to Washington."

Franklin, 42, a former judge and prosecutor from Greenwood, has made modest inroads within the black community with timely announcements of federal projects that would benefit black residents, including a new park and possible renovation of a hospital.

His $425,000 campaign budget, nearly twice that of Clark's, enabled Franklin to saturate the district with television commercials; Clark has had to settle for advertising on local black radio stations. Clark received a $25,000 contribution from the Democratic Congressional Campaign Committee, but most of that is in the form of in-kind services.

Franklin also stands to benefit from a large turnout by whites -- many of whom were registered this year to offset a voter registration drive by blacks -- while doubts linger as to whether Clark can turn out a large, unified black vote.

George Hooper, head of the Mississippi Voter Education Project and a staunch Clark supporter, contends that Clark is leading -- largely because of voter unhappiness over Reagan administration cuts in social spending. He said there is no way of telling, however, whether blacks will vote in large numbers this time.

Clark, 54, a veteran state legislator and a furniture store owner from Ebenezer, acknowledged the difficulties in challenging an incumbent in an interview following a candidates' forum here last weekend. He said his problems have been compounded by anti-abortion forces, who are upset about his pro-choice stance. Franklin has opposed federal funding of abortions.

"He Franklin has played upon the emotions of people," Clark said. "They sent one black fellow here to ask me about pro-life. He Franklin is all hung up on pro-life, and most folk is not pro-life at all. They are using that to play upon the emotions of people while starving the hell out of them."

Other black candidates also have faced difficulties in capitalizing on the excitement generated by Jackson's candidacy.

Freshman Rep. Katie Hall (D-Ind.), one of 21 members of the Congressional Black Caucus, was upset in a primary election May 9 by Peter Visclosky, who accused Hall of favoring black interests and neglecting white constituents. Gary Mayor Richard G. Hatcher, Hall's mentor, blamed her defeat on the failure of registered blacks to turn out in large numbers.

Simeon Golar, a black real estate developer, was expected to mount a strong challenge to veteran Rep. Joseph P. Addabbo (D-N.Y.) in a southern Queens district that had become 50 percent black. But Addabbo, in a rematch with Golar, picked up the endorsements of most local Democratic black leaders and amassed an impressive 67 percent of the Sept. 11 primary vote.

Israel M. Augustine Jr., a longtime black activist and former state appeals court judge, challenged Rep. Corinne C. (Lindy) Boggs (D-La.) after a remapping of the district increased the black voting population from 45 percent to 58 percent. Augustine campaigned largely on a theme of racial pride, but Boggs consistently had supported civil rights legislation and swept to victory with 60 percent of the all-important Sept. 29 vote.

So it went throughout the country, as black candidates for Congress time and again learned the limitations of rainbow politics.

Robert C. Smith, a political science professor at Howard University, said recently that Addabbo's victory in New York underscores a political fact of life -- that black voters are reluctant to oust a white incumbent who shares their general philosophical outlook in favor of a black candidate.

Addabbo, chairman of the defense appropriations subcommittee, has been instrumental in boosting the percentage of defense contracts earmarked for New York state.

An aide to D.C. Del. Walter E. Fauntroy, a former chairman of the Congressional Black Caucus, said recently that black voters frequently are caught in a "Catch-22 situation" in choosing between a white incumbent with seniority and an untested black challenger.

"It creates a situation where you ask, 'Do you vote these people out in favor of blacks and lose the seniority and power?' " said Julius Hobson Jr., the aide.

D.C. Mayor Marion Barry, who was a top adviser to Jackson's campaign, said recently that blacks may have erred by placing too much emphasis on presidential politics this year at the expense of congressional races. "In 1986, we should target 60 to 70 congressional districts and go after them," Barry said.

This year, the Congressional Black Caucus will be fortunate to retain 21 seats in the House in light of Hall's primary loss. The remaining 20 incumbents are thought to be safe, and black challengers may be able to pick up seats in South Carolina and Mississippi.

In South Carolina, Ken Mosely, a black physical education professor, won the Democratic primary in the 2nd Congressional District and, for the second time, will challenge seven-term Rep. Floyd Spence (R-S.C.) in the November election.

Mosely, who defeated Lt. Gov. Nancy Stevenson in the Democratic primary, is waging a well-financed and tough-talking campaign in which he has sharply criticized Spence for having accomplished little in Congress and for failing to prevent waste and mismanagement as a ranking member of the Armed Services Committee.

But there is little indication that Mosely has broadened his base of support beyond blacks and liberal whites, and Spence appears to be headed for reelection on a platform emphasizing constitutent service and the blessings of seniority.

In Mississippi's 2nd Congressional District, Franklin and Clark also are locked in a rematch. The odds shifted in Clark's favor early this year when a federal panel ordered a redrawing of district lines that increased the black share of the voting-age population from 48 to 53 percent -- although the percentage of registered blacks is somewhat lower.

Franklin, a conservative who supported President Reagan 76 percent of the time in 1983, and Clark, the liberal chairman of the Mississippi House Education Committee, provide voters with a remarkably clear-cut choice.

"Too many Mississippians for too long under too many federal programs are locked in a cycle of poverty," Franklin told about 200 people who attended the political forum last weekend. "I want us to break out, and I think the only way that we can do that is with forward-thinking growth through the private sector and less and less government control over our individual lives."

Clark, who has the backing of former governor William Winter, a candidate for the U.S. Senate, and several other key state Democratic Party leaders, has promised to fight for increased federal spending for job training for disadvantaged youths and to help rural areas ravaged by high unemployment.

Robert Gray, the mayor of Shelby, Miss., a black who ran against Clark in the primary, last week predicted a "very close race" in the general election.

"The key is going to be whether the blacks will come out," he said. "Over the years there has been so much voter apathy among blacks . . . . A lot depends too on the national ticket. Republicans are trying to create the image that it's over, but it's far from over."