Democrat Charles S. Robb, debating Republican J. Marshall Coleman yesterday in Vienna, accused Coleman of playing "an irresponsible game of politics" by pledging that as governor he would veto any tax increase approved by the Virginia legislature.

"That promise," Robb told more than 300 members of the Fairfax County Bar Association, "is simply too high a price to pay to get your vote."

Coleman defended his stand and charged that Robb is "becoming increasingly shrill" as he sees "victory slipping away" in the gubernatorial race.

The two men also traded charges about their respective ability to lead the state in what Coleman repeatedly called "the Reagan era."

Robb said "leadership is not . . . wrapping yourself in the coattails of a popular president" or "ignoring conflicts of interest so recognizable to everyone else, particularly to fellow lawyers" and failing to "build a broad coalition from all walks of life," including blacks, union members, teachers, conservative Democrats and the poor.

Coleman did not respond to Robb's attack on his role in the controversy over the location of the proposed Springfield Bypass, but said instead that Robb is "facing an identity crisis" in which "he gets upset at any reference to the Great Society, which made him a public figure."

Robb, son-in-law of the late President Lyndon B. Johnson, provided comic relief to the otherwise sharp exchange by giving Coleman a cowboy hat, which he said Coleman should wear the next time he goes to Texas.

"I haven't been to Texas as recently as you, Marshall. I haven't been there at all this year," Robb said, referring to Coleman's fund-raising expedition to Dallas and Houston last week. If he should return, Robb told Coleman, "when you go to the Petroleum Clubs (location of the Coleman receptions), the hat will improve your credibility, no matter which side you happen to be arguing."

Coleman refused to put on the hat, but glanced at the label and said it was from "the LBJ Co." He promised to give it to his son.

Coleman went on to make 11 references to the "Reagan era," and said that Virginia would not be well served by "a governor who must be dragged into the 80s and into the Reagan era." He said Robb was "the last Virginian to get on board" in support of the federal tax cut "and the first to jump ship when problems were not solved within 30 days."