Eighteen candidates who will be on the Arlington County ballot in November marked the start of the campaign season last night a a forum sponsored by the Arlington Civic Federation.

While candidates struggled with the time constraints imposed by the federation--two minutes to spell out their positions on a variety of issues--the 150 members of the audience sought to stay cool in the heat of the night.

One candidate quipped that the only people in the crowd who could be picked out easily were the male candidates: They were the ones in suits and ties.

After their brief opening remarks, the candidates responded to questions--often fielded by members of their opponents' camps.

As is usual in Arlington County politics, it is sometimes difficult to know which party a candidate belongs to.

For example, only one Republican will be on the November ballot--William Miller, who is challenging incumbent Democratic James F. Almand for the 47th District seat in the House of Delegates.

But the Democrats will have more opposition than that.

The other candidates who have won the Arlington Republican Party's endorsement are running as independents. All the Democrats are running as Democrats.

And from the audiences' queries, it appeared that the two races for County Board, traditionally the big draw for voters, may take a back seat to the sheriff and treasurer contests.

Republican-backed John E. Baber, who resigned from the Arlington County police force to challenge incumbent Sheriff James A. Gondles, a Democrat, scored points during the question-and-answer session.

Asked by a well-known Democratic partisan about a suit against him for alleged police brutality, Baber elicited cheers from his supporters in the crowd when he answered them the suit stemmed from his subduing a drunken driver.

Gondles, who is hospitalized, was represented at the forum by a deputy.

In another spirited exchange, Democrat Frank O'Leary, an economist, challenged the credentials of Republican-backed independent Dorothy T. Grotos, a two-term County Board member to be treasurer. Grotos countered that her years of dealing with county budgets, coupled with previous managerial experience, make her qualified for the post.

In the County Board races, Democrats Richard A. Buffum and Albert C. Eisenberg sparred with Republican-endorsed independents Walter L. Frankland Jr., an incumbent seeking a third term, and former school board member Michael E. Brunner.