Virginia Attorney General J. Marshall Coleman, the probable Republican nominee for governor, flew into the politcal backyard of his likely Democratic opponent yesterday to pledge the state's help in limiting air traffic at noisy National Airport.

Coleman's appearance with Fairfax Board Chairman John F. Herrity, one of the area's more colorful and popular Republicans, immediately raised the ire of many supporters of Coleman's expected Democratic foe, Lt. Gov. Charles S. Robb of McLean.

"Of course he's grandstanding," said Fairfax supervisor Martha V. Pennino, who is vice chairman of the Fairfax board and ranks among the area's most influential Democrats. "He knows this is Chuck Robb's home base, and he's trying to make an inroad up here. But he doesn't realize the public is going to figure it out."

Coleman, who traveled from Richmond to Dulles International Airport in a state-owned, twin-engine Piper Aztec, told a press conference that Virginia will intervene in support of the federal government in five pending lawsuits in which airlines are challenging Department of Transportation rules limiting air traffic at National. The regulations are scheduled to go into effect April 26.

Under the new rules, designed to reduce crowding at National And boost air traffic at flagging Dulles, National Airport would not be permitted to serve more than 17 million passengers annually. Without the regulations, federal transportation officials project traffic at the airport will soar to 19.2 million by 1990.

"We believe the regulations are in the public interest, and we're especially interested in the effect of the regulations, which will promote the greatest use of Dulles," said Coleman.

The smooth, self-assured Coleman, while acknowledging that airport limits are politically popular among noise-conscious Northern Virginians, maintained that his trip yesterday was not political in nature and said he had not yet decided when to reveal his political intentions.

"I've been wondering that myself," he deadpanned in response to a reporter's question. "What's your advice? I think I'll announce sometime between now and November."

But political insiders said the Fairfax announcement seemed to mark the first step in what is expected to be a hotly contested race between Coleman and Robb, a lawyer and son-in-law of the late President Lyndon B. Johnson, who draws on the populous Northern Virginia area for his support.

Northern Virginia has long been considered the keystone of any statewide political strategy, and recent Republican victories in the 8th and 10th congressional districts seem to offer the promise of a boost for Coleman in what had in the past been considered Robb terrritory.

"We're just happy he [Coleman] has discovered the issue," said a Robb aide yesterday, noting that the lieutenant governor long has been a supporter of proposed flight reductions at National. "We hope he can help out."

Fairfax Chairman Herrity denied that Coleman was using the airport issue for political gain, and maintained that the idea for the press conference had been his -- not Coleman's. "Everything a politician does is going to be considered political, whether it's a month or a day or three years before the next election," Herrity said. "But the bottom line that you have to consider is whether it will benefit the public good. And I think in this case it will."

Following Coleman's announcement, the Fairfax supervisors approved, 8-0, a measure pledging to throw their legal support behind the federal air traffic limits, and urging other area jurisdictions to do the same.

The Fairfax board last year had called for a ceiling of 15 million passengers annually for National, nearly two million fewer than that set by the Department of Transportation. Yesterday they agreed to back the federal regulations in an effort to insure the safety of passengers, reduce noise pollution, and increase the use of Dulles, located on the Fairfax-Loudoun country border.