When Maryland's Democratic delegates went to San Francisco to help nominate Walter F. Mondale and Geraldine Ferraro last summer, their ranks quickly were split in two during a bitter party platform fight.

Rep. Steny Hoyer of Prince George's County, a Mondale delegate from one of only two Maryland jurisdictions won by Jesse Jackson in the May 8 primary, immediately became a target for black Jackson delegates.

Hoyer's lack of support for Jackson's platform proposals, Jackson delegates warned, would cost him black support when he ran again for the House -- if not this year, then in 1986.

But Hoyer, 45, who is in the midst of a second reelection campaign, has good reason to believe that the threats will not come to pass. Prince George's County Democrats, fearful of what recent polls show as an increasingly likely Reagan victory in Maryland, are instead closing ranks this election year behind every Democrat in sight.

"Black people in Prince George's County have a formidable enemy in the person of Ronald Reagan," said Alvin Thornton, a Howard University political scientist and Jackson supporter who lives in Suitland. "There is a clear perception that he might win the county, and we don't want to do anything to help that."

The temporary lack of infighting is good news for Hoyer. Any lingering complaints about his leadership in the 5th District, he said, come from "factions," not from the black leaders he deals with.

John Ritchie, Hoyer's 41-year-old challenger, hopes that some of the lingering bitterness with Hoyer will help spill over into votes for him.

Ritchie has expended considerable shoe leather, but very little money, in his effort to unseat the two-term Democrat from Berkshire.

He estimates that he has spent 40 percent of his door-knocking, handshaking time in the county's black communities, hoping to tap into any evident well of unhappiness with Hoyer and perhaps turn up a little latent Republicanism as well.

Bennie Thayer, the local businessman who headed Jackson's state effort, does not think that Ritchie is likely to woo many Democratic voters who opted for Jesse Jackson just five months ago.

"We think Steny is going to win in 1984 with no problem, but we are also looking ahead to 1986 and a time when Steny Hoyer might not necessarily represent the 5th Congressional District," Thayer said.

Ritchie, a Laurel businessman who says he is a cousin to former Democratic Maryland governor Albert Ritchie, was soundly defeated in his last attempt for public office, a 1982 run against council member Frank Casula. This year, he is in search of the disenchanted.

"The Republican Party in this county has ignored the black vote, and I think the Democratic Party has taken it for granted," he said.

Hoyer, who says all of his fences are now mended, ponsored a breakfast for about 30 of the county's black elected officials and community leaders recently, and asked them to endorse his candidacy by signing a letter that will be sent to households in predominantly black areas of the district.

The letter cites Hoyer's "long distinguished record of working with the black community" and will be one of the few formal efforts Hoyer makes at campaigning for his own seat this year. Instead, he is concentrating on delivering votes for the Democratic presidential ticket, which includes his congressional buddy Ferraro.

"Geraldine Ferraro is a very close personal friend of mine who has said she will give personal attention to Prince George's County when she is vice president of the United States," he said in one stump speech.

But most of the time, he emphasizes his insider status in Congress as a member of the powerful Appropriations Committee. That position, he said, has enabled him to secure funding for such projects as the resurfacing of bumpy Suitland Parkway and the extension of the Metro Green Line.

Hoyer's congressional accomplishments have given him a reputation of being out front on issues that affect his constituents. But he continues to operate in the shadow of 8th District colleague Michael D. Barnes, who has gained a much more prestigious reputation in Democratic circles as an outspoken critic of Reagan administration policies.

Barnes, now serving his third term, has earned his rising-star reputation in part because he is chairman of an important Foreign Affairs subcommittee. Hoyer, however, is not yet identified as a key player on a national issue.

"You have to make a judgment on whether your member of Congress represents Prince George's well and has acted in a way you think is responsible and effective," he told a Laurel-area community group recently. "If I have not, then it's time to replace me."

Hoyer does not break a sweat when he says this. At the Laurel meeting, he was smooth and friendly, never unbuttoning his jacket or losing the timing he has developed in 15 years of state and national politics.

Ritchie, shedding his jacket, is more intense. In speeches and door-to-door, he describes Hoyer as a distant Capitol Hill politician who has neglected his constituents living just across the District line.

Ritchie spoke for several minutes about his commitment to people and his search for "common sense answers to everyday problems." Abortion, he said when questioned, should not be a political issue, school prayer should take the form of "30 seconds of reflection," and moral and religious beliefs, he said, do affect a legislator's role.

Hoyer told the crowd that the longer he stays in Congress, the stronger a voice Prince George's will have on Capitol Hill. He is prochoice, does not believe government should regulate prayer in schools and is opposed to what he terms the Jerry Falwell-type mixing of religion and politics.

Ritchie, door-to-door, identifies himself as Republican only when someone asks.

"Let's face it," Republican Party State Chairman Allan C. Levey said, "he's running in a district which is registered Democrat 3 1/2 to 1. He's going to get the Republican support. His major thrust has to be at Democrats."

That thrust is drastically underfunded, since Ritchie has gotten no direct financial support so far from either the state or national party. Ritchie's campaign treasurer, Joseph Kelly, said that as of Oct. 17 Ritchie had raised $7,800 for his entire campaign, far short of the campaign's $250,000 goal. Hoyer, according to federal election records, has raised $141,177 this year to add to the $106,000 he had already accumulated from previous campaign fund-raising.

Ritchie's treasurer said that most of his campaign funds have come from individuals who made small contributions. Hoyer's campaign funding disclosures show that he gets a large number of contributions from political action committees and unions.

"To my knowledge, we have not gotten much support from the Republican Party in the State of Maryland," King said.

But Levey admits that, although Ritchie's campaign is not the state party's priority, having a Republican standard-bearer actively campaigning in Prince George's has its benefits.

"He's helped the president's race by what he's doing," said Levey, who is optimistic about Reagan winning the heavily Democratic county. "John Ritchie will do better than anyone expects him to."

Said Ritchie, "I decided to run because I'm still convinced there was a need for somebody who was really concerned about the people -- not just the old political pablum. I've got the desire to help people. Now, that sounds corny, but it's true. I'm one that ends up tilting at windmills, even if it's against the odds."

Hoyer admits that he is not fighting terribly hard for his seat this year. A Hoyer congressional campaign, as such, may never gel.

"I go to three or four events three days a week anyway," he said one evening as he gathered with his staff at a union headquarters in New Carrollton to view the vice-presidential debate. "I frankly think the best campaigning is done each day, every day. I'm not in a tough campaign."