A loosely knit group of former Harvard classmates,Vietnam war veterans and South Dakota businessmen plans to become a presidential campaign committee when it flies this week as the Larry Pressler for President Committee.

Larry Pressler?

Pressler is South Dakota's 37-year-old freshman Republican U. S. senator, prairie farm boy, Rhodes scholar, Harvard-trained lawyer and Vietnam veteran. And to the friends and classmates who are promoting his candidacy, he is the only Republican who can beat President Carter in the 1980 election.

So far, Pressler disavows any plans to seek the Republican nomination in a growing field of contenders. But in a meeting yesterday Pressler did not make the "Shermanesque" statement that would take him out of the race definitely.

Rather, Pressler's stock reply is that "this presidential thing may not be in the cards right now,"and that seems to be enough to give encouragement to his small band of loyalists.

They met here yesterday at the home of one member to map a strategy for raising funds and persuading Pressler to run. This week will be devoted to "laying the foundation to encourage him to make that step," according to Paul Arneson, a Washington lawyer and leader of the drive. Arneson said part of that foundation will be the announcement of a formal campaign committee registered with the Federal Election Commission.

Pressler's supporters said they see the freshman senator as a "strong new Republican face," an alternative to the current GOP standard-bearers, As one group member said: Former California governor Ronald Reagan is "too old and too conservative," former Texas governor John Connally "has the milk thing." Sen Howard Baker of Tennessee flip-flopped on the Panama Canal treaties issue, "and nobody knows who George Bush is."

Pressler's backers readily admit that nobodty knows who Larry Pressler is either. As one Pressler loyalist remarked, "He has a good chance to be the vice presidential candidate, but you don't run for vice president,you run forthe top." CAPTION: Picture, Sen. Larry Pressler: It's "not in the cards."