Mrs. Kunin is still the favorite to win the election in November, according to political leaders and academic specialists here.

But the entry of Mr. Sanders, who is running as an independent, will siphon off some support from the left and could force the election into the state Legislature, these sources say. Under Vermont law, a candidate for governor must win over 50 percent of the vote or the contest is decided by a session of the state House and Senate.

The Republican candidate, Lieut. Gov. Peter P. Smith, is widely expected to draw fewer ballots than the popular Mrs. Kunin. But if the contest is thrown into the Legislature, where the Republicans hold a majority in the House and Democrats do in the Senate, the outcome is uncertain. Victory Narrow in 1984

In the last gubernatorial election in 1984, Mrs. Kunin narrowly defeated her Republican opponent, John Easton, the state Attorney General, winning 50.02 percent of the vote, or just 60 votes over the required majority. In the 1976 contest for lieutenant governor, the last contest in which a candidate failed to win over 50 percent, the Legislature chose the Republican though the Democrat won more votes.

The most recent statewide public opinion poll, released in January by the Becker Institute, reported that in a three-way contest, Mrs. Kunin would draw 56 percent of the vote, Mr. Smith 18 percent and Mr. Sanders 13 percent. Those data are considered out of date. Nevertheless, Mr. Sanders, a 45-year-old who still speaks with a strong hint of his Brooklyn upbringing, appears trapped. ''If he wins, he loses,'' said Garrison Nelson, an associate professor of political science at the Unversity of Vermont. Gloomy Outcome Foreseen