President Reagan and former Vermont Gov. Richard Snelling (R) have not always been the best of buddies. When Snelling was chairman of the National Governors' Association, his opposition to deep tax cuts in the face of growing federal deficits made him almost persona non grata at the Reagan White House.

But that has not stopped Reagan from taking a personal role this week in the effort to recruit Snelling as a challenger to Sen. Patrick J. Leahy (D-Vt.) in 1986. Snelling got a call from the president last Saturday, urging him to run, and accepted an invitation to discuss the race with Reagan at the White House on Wednesday.

Snelling, who said he told Reagan he still thinks a tax increase is essential to cut the deficit, will announce his decision on Thursday morning. The 58-year-old former governor, who is regarded as perhaps the only Republican with a chance to beat Leahy, still is wearing the speckled beard he grew for a recent transatlantic sailing expedition.

He has refused to tell even close political associates what he will do, but Republican strategists, who think the race could be pivotal in the fight for control of the Senate, are hoping that he would not be putting Reagan through the courtship ritual unless he were ready to run.

Snelling's only comment yesterday was, "Come to South Burlington Thursday and you'll be the first to know." Polls in the state show Leahy and Snelling almost dead-even in a hypothetical matchup.