Gov. Richard Snelling, in an ironic about-face spurred on by an old enemy, announced today that he will seek reelection this fall to an unprecedented fourth two-year term.

The 54-year-old Republican, chairman of the National Governors Association, said in October that he would retire when his present term expires next January.

Other Vermont Republicans, most notably Rep. James Jeffords, began organizing gubernatorial campaigns to fill the vacuum Snelling's departure would have created. Last week, however, those campaigns came to an abrupt halt.

Snelling's reversal resulted from a Feb. 2 editorial in the Burlington Free Press, Vermont's largest daily newspaper.

Ever since Snelling, a Harvard-educated industrialist, was first elected in 1976, the Free Press has been one of his most persistent critics, and the governor has not hesitated to express publicly his disdain for the newspaper.

But in last week's editorial the Free Press startled everyone, particularly Snelling, by urging Vermont Republicans to draft the governor for a reelection bid.

"Okay, so we don't always like Snelling's style," the Free Press wrote. "Nevertheless, we do respect him as a leader."

Snelling, who professed to be "totally amazed" by the paper's endorsement, responded by saying that if he perceived a "widespread and sincere" grass-roots desire that he remain, perhaps he would. For Republican leaders fearful of losing the state house to Democratic Lt. Gov. Madeleine Kunin, that was all they needed to jump on the draft-Snelling bandwagon, cranking out petitions and keeping the governor's telephone lines ringing off the hook.

In response to today's announcement, Jeffords declared that he would seek a fifth term in the House.

Many leading Republicans were reluctant to have Jeffords give up his safe House seat and return to Vermont as governor. The conservative wing of the party never has liked Jeffords, and was infuriated last year when he became the only Republican in the House to desert the president on the tax package. Even Snelling, a moderate, was not desirous of being succeeded by Jeffords.

As chairman of the National Governors Association, Snelling has been an exponent of the "new federalism" but he also has been critical of the way President Reagan has gone about forging the new federal-state relationship and has assailed Reagan's recent budget proposals.

Snelling has basked in the national attention generated by his criticism of the president, and many in Vermont believe this influenced his decision.