For more than a decade, the tiny but proud D.C. Statehood Party, founded by the late Julius Hobson Sr., has determinedly pressed what some say is its quixotic mission to make the capital city the 51st state.

Today, with its membership largely stagnant and statehood still a distant dream, the political party is facing a more immediate concern -- the invasion of Dennis Sobin and his "Sexcrats."

Sobin, a gadfly promoter of sexually explicit businesses, is poised to make a Lyndon LaRouche-style takeover of the party that claims fewer than 1,300 members in a city with more than 260,000 registered voters.

It's not the threat of LaRouche's extremist global politics that has angered longtime party members, but Sobin's unabashed goal of using the Statehood banner to field a slate of candidates in the Sept. 9 primary who have never been active in the party and simply want to tout their brand of sexual politics.

"I think we have to try to protect our party," said veteran Statehood member John A. Acher, a candidate for the Ward 1 council seat. "The party was started in the spirit of the '60s, the antiwar spirit, the civil rights spirit . . . . We are not the party of plutocrats, Democrats or Sexcrats."

Members of the Statehood Party, whose progressive platform includes liberalized planks concerning sexual behavior, have taken the unusual step of challenging Sobin's candidacy for mayor and his slate. A hearing on the matter before the D.C. Board of Elections and Ethics is scheduled for Friday. Party members contend Sobin's slate filed flawed campaign petitions and did not formally qualify to run in the primary.

"I see Sobin's move as political opportunism," said council member Hilda H. Mason (Statehood-At Large), a Hobson ally in the early years of the statehood movement and the titular head of the party.

Mason, the only Statehood Party member now holding office, is running for renomination. She said she resented Sobin's superficial interest in a "party that has devoted its whole life to very serious matters just to play a game . . . . I think it's unfortunate."

Mason said she and other longtime party members are working to keep Sobin from causing havoc similar to what happened earlier this year in Illinois, where LaRouche candidates took advantage of voter and major party apathy to win Democratic nominations for state offices.

"All of the members will know," said Mason. Based on previous primaries, a candidate in the Statehood primary would need only a few hundred votes citywide to win nomination to appear on the Nov. 4 general election ballot.

Sobin said yesterday he had begun distributing campaign literature to persuade at least 500 residents to register as Statehood Party members before the Aug. 11 deadline for registering to vote in the primary.

Sobin, who publishes a sexually explicit tabloid and is involved in several other sexually oriented enterprises, may be late for the election board hearing on Friday. Early that day, Sobin said he will be in D.C. Superior Court facing misdemeanor charges of operating a bawdy house. "They're coming at me from all directions," said Sobin.

Sobin, who previously has run for mayor as an independent, denounced efforts to exclude him last week, but acknowledged that he and his followers switched their voter registration from Democratic to Statehood solely to take advantage of easier qualifying requirements.

What made the little Statehood Party so inviting to Sobin is the city's unique home rule charter and the law governing which parties may automatically hold primaries.

In establishing the 13-member elected council in 1973 under a home rule charter, Congress sought to ensure minority party representation in an overwhelmingly Democratic city by saying that no more than two of the four at-large council seats could be held by any one party.

What's more, a political party retains the right to hold primary elections as long as at least one candidate from that party runs in every two-year election cycle and receives at least 7,500 votes in a general election. To appear on a primary ballot, a candidate must obtain the signatures of at least 1 percent of the registered voters in that party.

To run in the Democratic mayoral primary, Sobin would have had to obtain 2,000 signatures. But to qualify for the Statehood primary, Sobin needed only 13 signatures.

Some members of Sobin's slate who are running for council seats needed only one signature to appear on the Statehood primary ballot.

Council member Carol Schwartz (R-At Large), who is running unopposed for mayor in the Republican primary, needed only 213 valid GOP signatures to qualify for that party's primary.

Sobin switched his party registration on the last day of qualifying, along with several other candidates he supports, who have been dubbed the Sexcrats.

Jody Pappalardo, who operates an adult escort service, was a Sobin-backed candidate for the Ward 3 council seat. She was ruled off the ballot last week because she failed to switch parties before getting her nominating petition signed.

Others on the Sobin Statehood slate are Carolyn A. Wilson, a candidate for council chairman; Michael Jarboe, running for an at-large council seat, and Deborah Scott, a candidate for the Ward 6 council seat.

Hobson, the Statehood Party founder and champion of home rule, served on the first elected council and died in office in 1977. He was succeeded on the council by Mason.

Mason and Josephine Butler, a longtime civic activist, have carried the party's banner ever since.

Butler, the chairwoman of the Statehood Party, has run for several city offices in years when Mason was not on the ballot. She lost all those races but nonetheless received more than the minimum votes necessary to keep the party alive.

But now there is concern for the future, according to Butler, beyond the immediate problem presented by Sobin's slate. She said many of the party's activists are growing old and that she and Mason have begun discussions about how the party will continue its work into the 1990s.

"Hobson used to say we could hold a meeting in a telephone booth," Mason said. "There are some very good people who could run . . . . I'm sure there will be some."