PROVIDENCE, R.I. (AP) _ The newest Kennedy candidate is Sen. Edward M. Kennedy's youngest son, Patrick, and he's going after a seat in the Rhode Island Legislature although he's just a 21-year-old college student.

The 49-year-old Democrat who's held the job for 10 years resents it, and some key state Democrats say Kennedy is just practicing before he runs for Congress. He has to be 25 before he can serve there, however.

Kennedy, a junior at Providence College, a small, private school in the state's capital, is challenging Rep. John M. Skeffington Jr. for a part-time, two-year legislative post.

''I'm so surprised,'' said Skeffington, adding that he was an acquaintance of the Kennedy family. ''I just didn't think they would come in and run against an incumbent.''

''I am not opposed to Patrick Kennedy,'' said City Councilwoman Carolyn Brassil, the city Democratic chairwoman. ''I just feel that a student in Rhode Island shouldn't come into the state and run.''

''I get the impression that he's using this as a stepping stone to a congressional seat,'' said Brassil.

The Democratic primary in September includes a third candidate, city employee Angelo A. Pitassi. The winner will be unopposed in the November election because no Republican filed for the seat.

Skeffington had a difficult primary in 1986 and won renomination by only 185 votes against a 22-year-old political newcomer. The district has more than 4,900 registered voters.

Kennedy said the examples of his uncle, President John F. Kennedy, and his father, who won his U.S. Senate seat from Massachusetts in 1962, inspired him to get into politics.

''I've been given a name, and I think there's a certain responsibility that goes hand-in-hand with that name,'' he said. ''It would be easier to sit back and enjoy all the things I've been given. But I'm going to be spending my summer walking door-to-door.''

Skeffington questions the young candidate's commitment to the neighborhood after he graduates in two years, and calls attention to his family's wealth.

''I know that Pat has said on a radio talk show that he will spend as much as it takes,'' he said. ''I don't have those kind of funds. ... I'm going to run on my record.''

Kennedy spent $12,000 in the March primary to win election as a Michael Dukakis delegate to last week's Democratic National Convention.

Skeffington frequently uses the word ''they,'' meaning the Kennedy clan, when referring to his opponent, which Kennedy dislikes.

''People have said that the Kennedys have come in to take over,'' Kennedy said. ''That's no such thing. Patrick Kennedy is running for state rep.''

The newest Kennedy candidate faces several obstacles - his age, his relatively short residency and his name. He shares an apartment in a frame house in the Mount Pleasant district a few blocks from campus.

He's an outsider in the ethnic, blue-collar neighborhood, and a wealthy one at that. Despite his wealth, Kennedy says, his neighbors showered him with cards and expressions of sympathy when he was hospitalized last spring to have a benign tumor removed from his back.

Skeffington runs a funeral home that his grandfather opened after coming to America from Ireland in 1907.

''I am a Mount Pleasanter,'' said Skeffington, who turns 50 on Aug. 11. ''The people of my district are people like me. ... We're working-class people. We (politicians) try to do a good job for our constituents, and I feel I do a good job for them.

Kennedy said he watched Skeffington while working as a Statehouse page and legislative researcher his first year at college.

''I saw someone who has had a chance to do more for the community but hasn't done so,'' Kennedy said. ''I don't think an incumbent has a divine right to the office he holds just because he's an incumbent.''

Patrick Kennedy is the latest of a new generation of Kennedys to take a shot at politics. His brother, Teddy Jr., and sister, Kara, are running their father's re-election campaign in Massachusetts.

His cousin, Joseph P. Kennedy II, son of the late Robert F. Kennedy, is a congressman from Massachusetts. Kathleen Kennedy Townsend, the oldest daughter of Robert Kennedy, made an unsuccessful congressional bid in 1986.