In his 13 terms in Congress, Rep. Charles C. Diggs Jr. has never faced substantial political opposition in Michigan's predominantly black, overwhelmingly Democratic 13th District.

Diggs, an undertaker by profession, has a reputation here of burying his political challengers in heavy vote margins. He piled up a 4-to-1 landslide over the Republicans' last sacrificial lamb, Dovie T. Pickett.

But that 1978 landslide came a month after Diggs was convicted of 29 counts of defrauding the government of more than $60,000 in an elaborate payroll kickback scheme and receiving a three-year prison sentence.

Since then, the nation's senior black congressman has been censured by his House colleagues, stripped of his two committee chairmanships (House District Committee and the African affairs subcommittee) and, on Jan. 31, denied an appeal by the full nine-member U.S. Court of Appeals.

Diggs has steadfastly insisted he would seek reelection to a 14th term in Congress, and is confident his conviction will eventually be overturned. But this election year, Detroit's Democratic political establishment is starting to see Diggs as a liability, and five area politicians have announced plans to challenge the popular incumbent for the 13th District seat in the Aug. 5 Democratic primary.

For the first time in his political life, Charles Diggs may be in for a fight.

The latest candidate to announce a challenge to Diggs is Clyde Cleveland, a Detroit city councilman who came in third for the nine at large council seats in the 1977 city elections. Cleveland, 44, calls himself "the strongest candidate" to unseat Diggs, and calls it "a foregone conclusion that the congressman will not win."

In addition to Cleveland, Detroit City Councilman David Eberhard has announced plans to challenge Diggs for the nomination. The Rev. Nicholas Hood III, son of another Detroit city councilman, has likewise begun organizing his campaign against Diggs.

Another popular Detroit Democrat weighing a campaign for the 13th District seat is George Crockett Jr., 70, a retired recorder's court judge with widespread name recognition and a following in Detroit at least as strong as Diggs'. Crockett has said he will run for the seat only when it appears that Diggs' final appeal, to the U.S. Supreme Court, will be unsuccessful. t

Legal observers say it is possible the Supreme Court will decide whether to take the Diggs case in the spring term. The filing deadline for candidates in the 13th District is June 3.

Should Diggs' last avenue for appeal be denied and he be forced to begin serving his sentence, the race for the 13th District seat will be wide open for the first time in 26 years.

Diggs' indictment in 1978 produced a sympathetic backlash among his largely black, inner-city Detroit constituents. In the August 1978 Democratic primary, Diggs won renomination with 62 percent of the vote over three generally unknown political novices.

Diggs' challengers this year are quick to point out what when the incumbent won renomination, he had not then been convicted. Also, since three unknown opponents combined received 8,000 votes in the primary, Diggs' opponents see evidence that the incumbent's popular base, built up over two decades, may finally be eroding.

Diggs' challengers this time around are all well-known in the city, and are likely to force Diggs into active campaigning for the first time since winning his seat in 1954. But the conviction is unlikely to be an issue in the upcoming campaign, since Diggs has managed to rally his constitutency around the idea that the charges against him were at least in part racially motivated.

In fact, any challenger risks being buried under the same landslide Diggs piled up through sympathy in the 1978 primary. And in inner-city Detroit, the Democratic primary is more important than the general election.