Sen. Robert P. Griffin (R-Mich.), who narrowly missed being elected Senate minority leader last year and then announced his retirement, reversed himself yesterday by declaring he will run for reelection in November.

Griffin, who in April said he would retire for personal and family reasons, had been under intense pressure from state and national Republican leaders to run again and help the party keep the seat.

His reelection announcement in Traverse City, Mich., threw into turmoil the Senate race, as some Republican candidates already committed to run began dropping out.

However, the leading GOP contender, Rep. Philip E. Ruppe, scheduled a news conference for tomorrow to announce that he will remain in the race. Ruppe, who already has ruled out running for his House seat again, was "disappointed" by Griffin's announcement but is committed to going ahead, aides said.

Lt. Gov. James J. Damman was the first to withdraw.

William Seidman, an economic affairs adviser in the FOrd administration, is expected to drop out.

L. Brooks Patterson, a conservative and a prosecutor in the Oakland County suburbs of Derroit, declared his intention to remain. "I plan to challenge Sen. Griffin, who obviously entered the race under pressure from the political bosses . . . I plan to attack the bosses," Patterson said.

Griffin's announcement will give Michigan Republicans an extraordinarily strong ticket in November. Gov. William G. Milliken on Friday ended months of speculation about retiring and announced for reelection.

Milliken picked as his running mate James H. Brickley, president of Eastern Michigan University, in Ypsilanti, who from 1971 to 1974 was a popular lieutenant governor under Milliken.

Republican sources said Griffin changed his mind over the weekend, after meeting with Milliken here for a "long and quiet lunch" on Thursday.

The pressure on Griffin began in earnest in November, when a letter from the eight Republicans in Michigan's 19-member House delegation asked, in effect, that he consider the party's prospects first.

Among others who implored the former Republican whip in the Senate to run again were former President Ford and GOP National Chairman Bill Brock.

Griffin, 54, had said he wanted to return to Traverse City to spend more time with his family and to resume the law practice he gave up in 1956 when he was elected to the House.

Another widely accepted view was that Griffin's one-vote defeat by Howard H. Baker Jr. (Tenn.) for the Senate minority leader post in January 1977 was significant factor.