Michael D. Barnes, the overwhelming choice of Montgomery County Democrats to oppose 8th District Rep. Newton I. Steers Jr., outlined a computerized campaign strategy yesterday carefully designed to win the crucial support of independent voters in November.

"The most persuadable voters are the ones who don't care about party," Barnes said after getting 71.4 percent of the vote in a five-way race in Tuesday's party primary. "That's where the elelction is going to be won or lost."

Barnes, who began his campaign in March 1977, has already pinpointed what he considers to be the swing precincts through an analysis of party voting patterns throughout the county in the last 10 years. The precincts have been ranked from one to 171 based in the percentage of ticket-splittes, with the precincts with the highest percentage of splitters targetedto receive the most attention.

The problem with the strategy, according to Steers, is that "all but seven or eight precincts are swing precincts in Montgomery County. Montgomery County voters are extremely independent."

In large part because of that voter characteristic, the campaign between the 35-year-old Barnes, a lawyer who was formerly a Maryland public service commissioner, and the Republican incumbent Steers promises to be both hard fought and unpredictable.

Barnes, who cut his political teeth in the 1968 Muskie presidential campaign, has behind him a county Democratic Party more united than in years. Steers acknowledged that he was to some extent a beneficiary of Democratic divisions in 1976 when 11 candidates fought each other to oppose him. But it was 10 years ago, he recalled, that a Democrat nominated with 73 percent of the primary vote lost the general election to Steers'predecessor, Gilbert Gude.

Barnes plans to spend $80,000 to $100,000, mostly for television and radio advertising. Steers plans radio ads, but not television so far, and hopes to keep expenses to a "reasonable" level. At issues both men agree, is Steers' record, which is either "good" and progressive or "ineffective" and "irresponsible," depending on which one is describing it.

Maryland's 4th Congressional District also promises a lively race between four-term Rep. Marjorie S. Holt and Democratic challenger Sue Ward, a psychiatric social worker from Clinton who emerged from a three-way primary battle Tuesday with a 48 percent plurality.

Both women see it to an extent as an ideological battle between the conservative Holt and the liberal Ward. Ward said she anticipates an "extremely difficult" uphill battle. "I'm also battling a sort of push to the right we're (Americans are) undergoing nationally now."

Ward will also be battling to be the first Prince George's resident to win the district created in 1970 and dominated by Anne Arundel County voters. In Tuesday's primary, Ward won overwhelmingly in Prince George's precincts but lost in Anne Arundel to a resident of that county.

In the 5th District, located solely in Prince George's, Rep. Gladys N. Spellman won the Democratic nomination with nearly 83 percent of the vote. Her Republican opponent is Saul J. Harris, 55, a Landover Hills nuclear power lobbyist who described himself yesterday as "slightly euphoric" over winning his first political contest in Tuesday's primary.