OWINGS MILLS, MD. OCT. 29 -- Republican congressional challenger Wayne T. Gilchrest for the first time today openly used his record as a Marine combat veteran during the Vietnam War to challenge Rep. Roy P. Dyson's status as a conscientious objector.

"I went to Vietnam," Gilchrest, a Purple Heart recipient, snapped near the end of a testy half-hour debate filmed at Maryland Public Television's studios. "I fought so you had the freedom to be a CO . . . . Did you thank me?"

"Thank you, Wayne," Dyson said calmly. Dyson, a five-term incumbent and hawkish member of the House Armed Services Committee, avoided service during Vietnam through student deferrments and ultimately conscientious objector status; he says he felt the war was "immoral."

The exchange took place as Dyson recounted his support from veterans' groups, endorsements that visibly annoyed Gilchrest, who nearly defeated Dyson in 1988.

It was one of several heated moments as the candidates, irritated with each other and often talking at the same time, challenged each other on issues ranging from Social Security to revision of campaign finance laws. With the election only one week away and polls showing the race a toss-up, each wanted to stay on the offensive.

Dyson came armed with props to attack Gilchrest's stance on the federal budget, defense issues, and -- in a twist on an issue where Dyson typically has been on the defensive: fund-raising from political action committees.

Dyson has been dogged for two years by questions about the hundreds of thousand of dollars he has received from PACs connected to defense, agriculture and labor groups.

After Gilchrest charged that PACs had poisoned the political process and said politicians should not be allowed to take contributions from outside their districts, Dyson produced a fund-raising letter from the Gilchrest campaign he said was obtained from a PAC chief in Detroit.

"Detroit is not in Kent County," Dyson said, referring to Gilchrest's Eastern Shore home. The 1st District includes the Eastern Shore, Southern Maryland and part of Harford County north of Baltimore.

"To get in the ballpark you have to pay the ticket," said Gilchrest, referring to the high cost of running for election.

"Oh, you're going to take all you can," responded Dyson. "What ballpark? Memorial Stadium?"

Gilchrest said after the debate that the letter was sent to about 200 political action committees, most connected with business, whose causes he backs.

The two also sparred over the recent federal budget feud, with Dyson holding up stacks of mail from constituents urging him not to cut Social Security or increase Medicare premiums, steps Dyson says that Gilchrest would take if elected.

Gilchrest countered that Dyson has been irresponsible in refusing to vote for any of the proposed budget packages offered during the recent debate, while continuing to support defense spending and other programs. During the budget reduction discussions, Dyson has cited one program he would cut: foreign aid.