After eight months as a senator, Larry Pressler, of Humboldt, S.D., announced yesterday that he is a candidate for president.

Pressler, a 37-year-old Republican, said it is time to start electing presidents "in the prime of their lives" rather than the "twilight of his or her career.

"Many signers of the Declaration of Independence were my age or younger," he observed, his focus firmly fixed on a television camera.

In becoming the eighth candidate to formally declare for the Republican nomination, Pressler conceded he is "a long shot," with only a few thousand dollars in campaign funds and little organization.

He also indicated that he fears the presidency may be too big a job for anyone. "To announce for the presidency of the United States is a very ominous thing," he told a Capitol Hill news conference. "The job is so big that I don't think any one person can do it well."

Introduced as a "moderate conservative, Pressler said he plans to fashion a campaign around promoting gasohol and giving a better break to small business and farms.

He gave few specifics, however, on how he would carry out his campaign pledges. He did say that he would vote against the SALT II treaty in its present form, and would push for improved benefits to Vietnam veterans. Pressler is the only senator to have served in Vietnam.

First elected in 1974 to the House, Pressler has been one of South Dakota's most popular officeholders. But the home-state reaction to his presidential candidacy is far from enthusiastic.

Informed of it, Republicans state chairman Dan Parrish said, "My reaction in three words would be ha, ha, had." I just really can't take his candidacy seriously myself. I probably shouldn't say that as chairman of the South Dakota party."

Parrish later said he thought the reporter on the phone was making a "crank call." But he did not recant his statements.

"He hasn't got a chance," said Gov. Bill Jacklow, a Republican. "The election of a president is a very serious thing in America, not to be taken lightly. He has no chance at all of being elected. He knows it, you know it, I know it. He isn't going anyplace . . . I wish him well."

Pressler was elected to the Senate last fall after two terms in the House. He holds a law and graduate degree in government from Harvard and was a Rhodes Scholar. After serving as an Army lieutenant in Vietnam he was a State Department lawyer.