Sen Robert P. Griffin (R-Mich.), who announced in April he will not seek re-election next year, has come under intensive pressure from state and national Republican leaders to reconsider and help the party keep the seat by running again.

Among those imploring Griffin, who is 54, to run for third term have been former President Ford, Gov. William G. Milliken, Republican National Committee Chairman Bill Brock, and the Republican congressional delegation from Michigan.

While Griffin's public posture has remained firm against running again, Republican sources said the former minority whip has become increasingly sensitive to the party officials' distress over the lack of electable candidates willing to run for his Senate seat.

Milliken said last month he would not seek the Senate nomination, and Rep. Guy Vander Jagt (R-Mich.) similarly took himself out of the race in October, leaving the party with a field of potential candidates with no demonstrable attraction for carrying the state in a Senate race.

The disclosure Tuesday by Rep. Phillip E. Ruppe (R-Minch.) that he will formally announce his candidacy for the Senate in January presumably will take some of the pressure off Griffin to run, although party tacticians said yesterday the impact of Ruppe's announcement has yet to be assessed fully.

Ruppe, who has served in Congress since 1967, is popular in his sprawling Upper Peninsula district, which geographically covers a third of the state, but he is not well known in the populous southern part of Michigan, including Detroit.

The pressure on Griffin to rescind his retirement pledge began in November when Michigan's eight GOP congressmen sent him a letter asking, in effect, that he consider the party's prospects first.

Griffin had said he wanted to return to his home in Traverse City, Mich. for personal and family reasons, and to ressume 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 last January by Sen. Howard H. Baker Jr. (Tenn.) for the Republican leader post was a significant factor.

Michigan Republican leaders said appeals to Griffin to run were based, in parts, on Griffin's own assertions when he announced his retirement that he fully expected such leading Republicans as Milliken, Vander Jagt or Ruppe to get into the race.

Whether the leaders continue to press Griffin to run will be indicative of how much confidence they have in Ruppe to carry a statewide election.

The only announced Republican candidates for Griffin's seat are Lt. Gov. James J. Damman and Hillsdale College President George Roche. The most frequently mentioned Democratic candidate is Rep. William D. Ford, who represents the blue-collar suburbs of Detroit.