Former governor Richard Snelling said yesterday he will run for the U.S. Senate next year against incumbent Democrat Patrick J. Leahy, setting up a contest between Vermont's two most popular politicians.

Snelling said lack of action to erase the federal deficit provided the prime motivation for his candidacy, and he said Leahy "has not shown any willingness to get the deficit under control."

Snelling had been under pressure from national and state Republicans to enter the race. President Reagan joined the effort Wednesday, asking Snelling to run in a year that the GOP fears it may lose control of the Senate.

Snelling stepped down in January after eight years as governor, the first person in Vermont to be elected to four two-year terms.

Former Reagan administration Transportation secretary Drew Lewis has let friends and supporters know he's interested in running for governor in Pennsylvania next year, but only if he doesn't have to be the one to crowd out Lt. Gov. William Scranton III, whose political career Lewis helped launch.

So some of Lewis supporters are trying to do the dirty deed for him. They are spreading the word that Scranton -- who has been preparing for the race for six years and has already banked more than $1 million -- will be a weak candidate. Lewis, chairman and chief executive office of Warner-Amex Inc., met with political supporters last week and chose to let the anti-Scranton wave build. If it gets big enough, he's poised to ride it into a candidacy.

Meanwhile, there are strains elsewhere on the Pennsylvania GOP front. Gov. Richard L. Thornburgh (R), prohibited from seeking reelection to a third term, seems to be edging closer to a primary fight against Sen. Arlen Specter next year. The governor has a poll, taken by Richard Wirthlin, that shows him beating Specter by 16 points in a primary; Specter's poll, taken by Robert Teeter, has Thornburgh up by 12.

Specter's efforts to head off a Thornburgh candidacy have only piqued the governor's interest. "If they hadn't been so heavy-handed, we may not have gotten this far," says Thornburgh press secretary David Runkel. A decision is expected by mid-November.

South Carolina's two young Republican congressmen -- Reps. Carroll A. Campbell Jr. of Greenville and Thomas F. Hartnett of Charleston -- have been on a collision course all year. Both have said they plan to retire from Congress in 1986 to run for governor. For months, party leaders have tried to persuade one or the other to go after Sen. Ernest F. Hollings (D-S.C.) instead, arguing that having both men on the ticket would avoid a divisive primary and strengthen GOP chances in both the gubernatorial and Senate races.

So far, neither Hartnett nor Campbell has budged. Now it's let's-see-who-blinks time. The National Republican Senatorial Committee has just completed a poll that shows Campbell defeating Hartnett in a GOP gubernatorial primary by 2 to 1. Armed with those numbers, Campbell forces are stepping up their efforts to move Hartnett into the Senate race. Something is expected to break within a few weeks. -- Paul Taylor