Democratic gubernatorial candidate Charles S. Robb took the offensive against Republican opponent J. Marshall Coleman today, criticizing Coleman for wrapping himself in President Reagan's coattails and accusing him of making irresponsible campaign promises on taxes and other issues.

"Virginians want a governor who can think for himself and whose focus will be on Virginia and not Washington," Robb told a sparse crowd gathered in Capitol Square for the official kickoff of the Democrats' fall campaign. "The fact of the matter is, it will be Chuck Robb or Marshall Coleman, not President Reagan . . . standing on the steps here next January and taking the oath of office."

Robb said that while he opposed tax increases and favored cutting government costs, he could not promise -- as Coleman has -- to veto any tax increase passed by the Virginia legislature. "I believe such a position would be irresponsible and it's too high a price to pay for your vote," said Robb, the state's lieutenant governor. He also labeled Coleman's proposed changes in sentencing of convicted felons "slick phrases and empty rhetoric."

The direct attacks on Coleman were a departure from Robb's customary style of avoiding even the mention of his opponent's name. Robb denied afterward that the attack was a major change in strategy, saying he did not plan to attack Coleman personally but adding, "If the shoe happens to fit, anybody can wear it."

Coleman, the state's attorney general, was campaigning in the Tidewater area today, and did not respond directly to Robb. But his press aide, David Blee, maintained Robb's offensive "confirms what we see in the polls -- signs of an increasingly desperate campaign that's flailing."

Blee noted that in recent weeks Robb had been relatively silent about Coleman, while others such as former state appropriations chairman Roy Smith and former attorney general Andrew P. Miller have attacked the Republican nominee. Miller and Democratic attorney general candidate Gerald Baliles have both blasted Coleman this week for refusing to resign from his state office while campaigning for governor, saying Coleman's part-time status had forced the state to hire expensive outside counsel to defend the General Assembly's reapportionment plans in court.

"It's refreshing at least to see him Robb attack us himself instead of using surrogates to do his dirty work for him," said Blee. He added that Coleman, who has been taking half-pay since the spring, had no intention of "turning his back on his oath of office . . . just to placate a few Democratic politicians trying to make political hay."

In the past, Robb had avoided direct criticism of Coleman on the resignation issue, but in an interview after his speech today he said, "It's fairly obvious what course Coleman ought to take . . . It's a little difficult to serve the people only at your convenience."

Baliles and Democratic lieutenant gubernatorial candidate Richard J. Davis joined in today's offensive. Baliles said of Coleman and his Republican running mates, "You won't find a more erratic course taken in this century by a statewide ticket." Davis, referring to conflict-of-interest allegations against his GOP opponent, state Sen. Nathan H. Miller, said, "None of us are going to wear two hats while we're in office."