Sue V. Mills, for a decade the symbol of middle-class white opposition to court-ordered busing in Prince George's County, yesterday entered the special Democratic congressional primary, saying she would best serve the sonstituents of the 5th Congressional District.

Mills does not live in the district. She lives in Oxon Hill, in the 4th Congressional District. But that, she said, is not a problem. "I'm the only candidate who represents all the people of the 5th Congressional District," she said at a press conference in New Carrollton, noting that she often represents residents of the 5th district as a member of the county council.

The results of the 1978 council election seem to support her claim of popularity among voters in the 5th district, which encompasses the northern section of Prince George's County. In that district and throughout the county, Mills captured more votes than any other candidates except Frank Francois. The only neighborhoods within the 5th that she did not run away with were the mostly black areas of Seat Pleasant and Landover, where she ran fifth.

Mills, 45, who served on the school board for eight years before she was elected to the county council, said she decided to run for former Congresswoman Gladys Spellman's seat because "If I don't run, the Democratic voters will have no alternative but to select someone who is for big spending, high taxation and waste."

Mills did not name programs she would cut to save money. Instead, she talked about certain "worthwhile" but endangered programs that she would fight to preserve.

First on her list was social security, "not a welfare program or government handout." Second was federal impact aid for education, of which she said the county is not getting enough. "That's federal money that should come into Prince George's County and we're going to lose it," she said. Finally, she mentioned student loans, which would be "counterproductive to cut." Mills suggested that the Internal Revenue Service collect the loan money from former students because "everyone knows that God and the IRS never lose track of anyone."

Mills, who defeated a candidate of the dominant Democratic organization in the 1978 election, said she gets a "kick" out of serving constituents. "People have all sorts of problems they call me with," she said. "They have broken sidewalks, or they want to know if they can have rabbits in their back yard, or they're having a problem transferring their kids to another school. I like to fight through the maze to help them."

Mills refused to size up her opponents for the Democratic primary or the election, saying, "I don't run against anybody, I run for a job."

The only person she compared herself with was the former congresswoman whose seat she is seeking, Gladys Spellman. Mills mentioned that she and Spellman have similar views on the need for serving constituents, and for more impact aid to education and for helping federal workers.

Then she drew the comparison further. "Our careers have been pretty much along the same path," she said adding that both she and Spellman gained popularity while working on the PTA and that both served on the county council.

Mills did not mention that Spellman's supporters tend to be more liberal than hers. While Mills' supporters were fighting against busing, Spellman's were fighting for it.