State Sen. James Clark (D-Howard), a fixture in the General Assembly for 28 years and former president of the state Senate, has announced that he will not run for reelection this fall.

Clark, 67, who represents most of Howard County and part of Montgomery, said it was "time for me to leave the legislature" but did not rule out running later for another office.

Clark was Senate president from 1979 to 1983, when he was ousted in a party battle by Sen. Melvin Steinberg of Baltimore County. A dairy farmer, he was elected to the House of Delegates in 1958 and the state Senate in 1962.

Although he has indicated that he would prefer a Howard County resident, Clark said he will not endorse any candidate for his seat.

Reviewing his legislative career, Clark said he was proud of his drive to reform the state employe pension system and of his early stand for civil rights in the mid-1960s. "I wielded a lot of power, but I always did it quietly," Clark said. Sullivan Media Push Set

Ken Smith, campaign manager for Republican Senate candidate Richard P. Sullivan, said the Sullivan campaign is launching a $100,000 publicity blitz this week.

Targeted are about 60,000 Maryland households that have at least two registered Republicans. Each of those households will get a three-page mailer on Sullivan, a Baltimore businessman, followed about a week later by a fund-raising letter. In all, about 177,000 pieces of mail will be sent out at a cost of $60,000, Smith said.

At the same time, Sullivan will run televised advertisements for a week in Baltimore, Salisbury and Hagerstown, Smith said. The ads, which include a picture of Baltimore Mayor William Donald Schaefer with Sullivan, feature Sullivan at the Port of Baltimore talking about his "real experience" with issues such as trade and jobs. Two Quit Montgomery Races

Two Montgomery County Democrats who were seeking countywide political offices have abruptly abandoned their campaigns three months before the party's September primary.

Jay S. Bernstein, who failed to win a spot on a ticket of County Council candidates despite his status as local Democratic Party chairman, ended his campaign last week. He said he dropped out because he faced a simple fact of political life in Montgomery: "You cannot run without a slate."

Meanwhile, Tony Puca, who until recently maintained that he would run for county executive, dropped out of that race even before filing the necessary paperwork. "Business and personal commitments would make it impossible for me to serve as county executive," he said.

But Puca does intend to run for state Senate. In announcing his retreat from the executive race, he said he would repeat his challenge against Laurence Levitan, the senator for legislative District 15, which includes the western half of Montgomery.

The entry of Puca, who waged a campaign against Levitan in 1982, should make for a long, hot summer for the incumbent. Levitan also faces Steve Leas in the primary; the winner will face Republican Robin Ficker.