David L. Scull, a Montgomery County Council member, lawyer and scion of one of the county's oldest political families, will formally announce his candidacy for county executive this week, a move that promises to make the race for that post hotly contested next year.

The 42-year-old Democrat has been indicating his interest in the county's top office since June, but also was mentioned as a possible candidate for Congress after Rep. Michael D. Barnes (D-Md.) announced plans to run for the U.S. Senate.

"The nation's problems are certainly challenging, but I concluded that I could make my largest contribution as executive," Scull said yesterday in a telephone interview.

Scull said he will make the announcement Saturday at a political rally at the former Silver Spring home of his late parents, who both served on the County Council.

He will become the third candidate to seek the Democratic nomination for executive.

State Sen. Sidney Kramer (D-Silver Spring), and county recreation department director Dave Robbins also are in the race; still other candidates may emerge during the next few weeks, party officials indicated.

Kramer, a self-made millionaire who served on the council and ran unsuccessfully for Congress in 1976, was endorsed earlier this year by County Executive Charles W. Gilchrist. Gilchrist has announced that he is leaving his post at the end of the term.

While Democrats hold the edge in registration, a crowded and bitter primary could benefit a Republican candidate, said Allan C. Levey, the state GOP chairman.

"People in Montgomery County don't like negative campaigns, and I think a race between Kramer and Scull will be very negative," Levey said.

Edward Gannon, an economic consultant who worked in 1982 for Republican Joseph C. McGrath during his bid for county executive, is the only candidate currently seeking the GOP nomination, Levey said.

Scull considered challenging Gilchrist when he ran again in 1982, but instead headed a slate of candidates that won a 4-to-3 council majority over a rival slate backed by the executive. Scull won a seat in the Maryland House of Delegates in 1974 and served for seven years, rising to become chairman of the county's 19-member House delegation.

He was appointed in 1981 to fill a council seat vacated by the death of his mother, Elizabeth Lee Scull, sister of former acting governor Blair Lee III and member of a politically powerful family whose roots in the county go back to the Revolutionary War.

Scull's father, David, a Republican, served on the council in the 1960s. His grandfather, Col. E. Brooke Lee, was the county's undisputed political boss for three decades and built a fortune through local real estate development.

Scull said he sold his interest in the family's real estate holdings last March to avoid any conflict of interest with his public duties.

After graduating from Princeton and the University of Virginia Law School, Scull worked in a law firm headed by R. Sargent Shriver and Max Kampleman, now President Reagan's chief arms negotiator and in a public-interest law firm funded by liberal philanthropist Stewart Mott.