STROM THURMOND is a political activist. Elected governor of South Carolina in 1946, he ran for president, two years later, on the States Rights ticket. Elected to the Senate as a write-in candidate and a Democrat, he became a Republican in 1964 to help his philosophical soul mate, Sen. Barry Goldwater, capture the presidential nomination and carry South Carolina that fall. Richard Nixon, in 1968, made the courting of Sen. Thurmond one of the first priorities in his successful Southern strategy. Mr. Thurmond's support of Mr. Nixon that year was crucial to stopping Gov. mRonald Reagan's challenge and winning the first-ballot nomination for Mr. Nixon. In 1976, the senator endorsed Mr. Reagan over President Ford in the Republican presidential contest. Last week in Columbia, S.C., Mr. Thurmond, a prominent veteran of at least nine national campaigns, endorsed former Texas governor John Connally for president.

South Carolina Republicans will hold a presidential primary, the state's first, on March 8. The date is significant, because it follows primaries in New Hampshire and Massachusetts, where Mr. Connally has all but conceded he will not defeat the Republican front-runner, Ronald Reagan. The South Carolina primary will take place only three days before primaries in Florida, Georgia and Alabama. Mr. Connally has frankly acknowledged that he would have to beat Mr. Reagan fairly early in the primary season to have any realistic chance of being nominated. That brings us to the endorsement. Mr. Reagan had already named a campaign chairman in South Carolina, freshman Republican Rep. Carroll Campbell. Mr. Campbell has occasionally indicated a more than passing interest in Strom Thurmond's Senate seat, which will be up for election in 1984. There is also a generation gap of sorts to further complicate the presidential picture in South Carolina. Mr. Campbell is 39 years old; he was in the second grade when Strom Thurmond ran for president. Sen. Thurmond turned 77 on Dec. 5.

The old and honored political axiom has it that "all business is local." Presidential endorsements in South Carolina suggest that this old axiom may still be valid. Mr. Reagan, who will observe (if not celebrate) his 69th birthday next month, chose the youthful Mr. Campbell as his South Carolina chairman. This enabled Mr. Connally to woo and win Sen. Thurmond. In the politics of 1980, it is very unlikely that any endorsement really translates into significant voter support. The vote for president tends to be a personal decision involving highly subjective factors. Even so, with Strom Thurmond's established record as an able reader of political currents and breezes (especially in South Carolina), the news of the Thurmond endorsement is the best news John Connally has had in quite a while.