Rep. Joseph L. Fisher began the first debate with his Republican opponent last week by telling members of the Arlington Chamber of Commerce that the race in the 10th Congressional District of Virginia pits "a conservative against a moderate."

"I was interested to hear Joe refer to himself as a moderate," said Fisher's challenger, Frank R. Wolf, as he held up a news clipping from The Washington Post. That article, Wolf continued, "said Joe is a solid liberal . . . Joe, I don't think you ought to be embarrassed by you record. I just think those are the facts and where you are."

That encounter, 154 days before the election Nov. 7, sets the stage for a campaign between two men whose campaign style is as different as night and day.

The 64-year-old, white-haired Fisher is a reserved, scholarly type who answers questions in a slow, moderate tone as if he's searching for the right word.

Meanwhile, the 39-year-old Wolf is highly aggressive, often leaping to respond to Fisher.

"I think I'll get to set the pace," said Wold, who already has gathered nearly two times more in campaign contributions than Fisher.

"He's the incumbent," Wolf said. "I'm going to have to take the issue to him."

Wolf said he has asked Fisher to participate in five more debates.

"I think it's very hard to hold it to five debates," said Fisher, who in his 1976 campaign debated his oppenent, state Del. Vincent F. Callahan, dozens of times.

The 1976 debates represented the first time in at least 24 years that voters in the 10th District has the opportunity to hear the incumbent confront his challengers.

Former Republican Rep. Joel T. Broyhill, who was unseated by Fisher in 1974 after serving 22 years in Congress, went so far in refusing to confront his opponents that even television interviews were taped separately.

At the debate last week, Fisher stressed his constitutent service. He mentioned the 50 to 80 town meetings he has held since being in office and the congressional offices that have been set up through the district.

Meanwhile, Wolf emphasized his support of major cuts in the federal tax rate.

He said he favors the completion of the Metrorail system. (He took out a Metro farecard from his pocket to show that he had used the subway to travel to and from the debate, held at the Stouffer's Inn in Crystal City).

Fisher, an economist, is a member of the powerful House Ways and Means Committee, which local politicians view as a plus in his reelection bid.

Wolf, who lives in Fairfax County and practices law in Rosslyn, is a former congressional aide and a deputy assistant secretary for congressional relaions in the Interior Department during the Nixon and Ford administrations. Two years ago, he lost the Republican primary for the congressional seat to state Del. Vincent F. Callahan. Callahan then was defeated by Fisher in the general election.

Of last week's debate, Fisher said, "I think it's too early to register very much with people."

Wolf said, however, he didn't think the debate was held too early in the campaign. "I've been out since March. . . . I'm second and I've got to work harder."