Former Arizona governor Evan Mecham (R) conceded defeat in his bid for a political comeback yesterday, but elsewhere incumbents and ex-incumbents had an easy time as 10 states and the District of Columbia held primaries on the busiest day of balloting so far this year.

Former Vermont governor Richard A. Snelling won the Republican nomination for his old job while Republican Govs. Tommy G. Thompson of Wisconsin, Edward Di Prete of Rhode Island and Judd Gregg of New Hampshire were renominated. In Minnesota, Gov. Rudy Perpich (D) also won renomination in a come-from-behind campaign.

In New Hampshire, one former U.S. senator, John Durkin (D), defeated two rivals for nomination to his old job, and one current senator who has chosen to leave office after two terms, Gordon J. Humphrey (R), was nominated to a $100-a-year state Senate seat.

Two congressmen seeking higher office took a first step yesterday: Rep. Robert C. Smith brushed aside three opponents to win the GOP Senate nomination in New Hampshire while in nearby Connecticut, Rep. Bruce A. Morrison overwhelmed state Rep. William Cibes to win the Democratic gubernatorial nomination.

Mecham, who was impeached in 1988 after two tumultuous years in office, lost by a ratio of nearly 2 to 1 to developer J. Fife Symington III, who will face former Phoenix mayor Terry Goddard (D).

A summary of the races:

Connecticut. After winning the state's first Democratic primary for governor in 12 years, Morrison will face former U.S. senator Lowell P. Weicker Jr. (R), who is running as an independent, and the GOP nominee, Rep. John G. Rowland, who had no primary opposition. Gov. William A. O'Neill (D) is not seeking a third term.

Morrison painted Cibes as an O'Neill crony and a proponent of tax increases, while Cibes criticized Morrison for not presenting a plan to deal with the state's slump.

Minnesota. Earlier this year, Newsweek dubbed Perpich "Governor Goofy" for an unorthodox personal style that has led him, among other things, to threaten to sell the governor's mansion after he was criticized for its $2 million renovation.

Perpich, a 10-year incumbent who has served longer than any governor in state history, trailed in the polls all summer but began turning things around by touting a rebound in the state's economy. He defeated one of his appointees, former state commerce commissioner Mike Hatch, who contrasted his abortion rights stance with Perpich's antiabortion stand.

In the GOP primary, businessman Jon Grunseth, an antiabortion activist and the party-endorsed candidate, upset state auditor Arne Carlson, who supported abortion rights.

In the Democratic Senate primary, Paul Wellstone, a political science professor who ran Jesse L. Jackson's 1988 presidential nomination campaign, won the party nod over state Agriculture Commissioner Jim Nichols. Wellstone will face Sen. Rudy Boschwitz (R).

New Hampshire. Humphrey, keeping a pledge to leave the U.S. Senate after two terms, easily overcame a challenge from state Rep. Jack Sherburne, a retired Air Force fighter pilot, in the race for the state Senate.

In the Democratic race for the Senate, Durkin, who was defeated in 1980 by Warren B. Rudman (R), won nomination over Nashua Mayor Jim Donchess and former businessman John Rauh. In the Democratic gubernatorial primary, former state party chairman Joseph Grandmaison had a narrow lead over state Senate Minority Leader Robert Preston.

New York. Carol Bellamy, the former New York City Council president who ran unsuccessfully for mayor in 1985, won the Democratic nomination for state comptroller against Westchester County Clerk Andrew Spano. She will face incumbent Edward Regan (R) in the Nov. 6 election.

Rhode Island. With the state's economy in trouble and questions lingering about land deals, Di Prete figures to be one of the nation's most vulnerable executives. In November, he again will face businessman Bruce G. Sundlun (D), the Democratic nominee in 1988 and 1986. Sundlun won yesterday's primary over Warwick Mayor Francis X. Flaherty, with Providence Mayor Joseph R. Paolino Jr. running well behind.

Vermont. Snelling, who was governor from 1977 to 1985, defeated Republican challenger Richard Gottlieb and is favored over Democrat Peter Welch, the former president pro tem of the state Senate, in the November election. Gov. Madeleine M. Kunin (D) chose to retire.

Wisconsin. Thompson defeated Ben Masel, a marijuana-legalization activist, setting up a race in November against Tom Loftus (D), speaker of the state House, who was unopposed.

Staff researcher Bruce Brown contributed to this report.