When Vincent F. Callahan Jr. first started out in Virginia politics back in the early '60s, one of his first moves was to join the Republican Party. "I just thought that Virginia was a backward state," said Callahan, a native of the District, "and that the Republican Party was where the opportunity was and where the progressive ideas were."

Now Callahan, a nine-term Republican delegate from McLean and House minority leader in Richmond, is in a race where his party is a key issue. Democratic contender Joseph W. McDonald, a 30-year-old political consultant, argues that in becoming the GOP's chief spokesman in the legislature, Callahan has forsaken his constituents in the 34th House District.

"He is often locked into a position as leader of a Republican Party that is becoming increasingly right wing," says McDonald, now trying for the second time to unseat Callahan. "What this means is that his priorities are established by the Republican Party, rather than by the needs of the district."

Although the 34th House District, which includes McLean and Falls Church, is considered one of Northern Virginia's wealthiest, there are people who don't fit the comfortable suburban mold, says McDonald. "What it all comes down to is whether Vince Callahan represents all the people, and I maintain that he represents a very narrow, closed group," says McDonald.

McDonald has suggested a conflict of interest in votes cast by Callahan, a director of the McLean Savings and Loan Association, on banking bills that come before one of his committees. And he criticizes Calla- han's votes against state funds for at-home care for the elderly, against lower auto insurance premiums for those over 55 and against a bill two years ago that increased fees for marriage licenses to pay for shelters for victims of spouse abuse.

Callahan sees no conflict in votes on savings and loans issues because, he says, most of the bills are simply technical measures to make state laws conform with federal law. As for the others, Callahan said: "If that's the best he can come up with, he's really nitpicking. He might have voted the same way I did if he had heard both sides." Callahan, for instance, advocated using general state revenues for spouse-abuse shelters.

Unflappable in his campaign as he is on the floor of the House, Callahan makes no apologies for achieving a ranking position within his own party. As leader of the 34 Republicans in the 100-member House--he was elected in the caucus by a wide margin in 1982--Callahan says he now has more influence to exert on behalf of Northern Virginia. Already a member of the Appropriations Committee, Callahan was recently named to the Committee on Rules and to the Joint Legislative Audit Review Commission in his capacity as minority leader.

When the governor gathers the legislative leadership together for private briefings, Callahan is the only Northern Virginian in the group "not in spite of being a Republican, but because I am a Republican," he says.

Callahan generally gets high marks for his performance as minority leader, a role he plays in his usual low-key style. Colleagues cite his efforts to keep the caucus informed and coordinate its efforts with Senate Republicans. But mostly, they say he knows when not to turn votes into partisan battles. "He doesn't make a political issue out of everything that comes down the pike," said Del. Clinton Miller (R-Shenandoah).

There are few party line votes in Richmond, maybe four out of an average of 1,200 cast a year, according to Del. Robert Harris (R-Fairfax). "Most votes come down along liberal-conservative lines or along regional lines," Harris said. "Mostly, we in the Northern Virginia delegation are cast in the roles of liberals."

Callahan, who lost a 1965 race for lieutenant governor on a ticket with GOP moderate Linwood Holton and a 1976 congressional contest against former representative Joseph Fisher, said his string of legislative victories backs up his claim that he hasn't lost touch with his constituency. In his last eight legislatives races, Callahan led the ticket in the precincts that now make up the 34th District. Last year, his first time out in a single-member district, he beat McDonald with 60.4 percent of the vote.

McDonald, an Arlington native and graduate of American University who lives in Falls Church where the district's Democratic voters are concentrated, expects to spend $5,000 against Callahan's budget of $18,000. He got a boost last week from Gov. Charles S. Robb at a fund-raiser attended by 80 people.

McDonald, who has worked in campaigns for State Sen. Clive DuVal II and for Fisher in 1980, sees his effort last year as a warm-up for an upset this year. "We set 40 percent as our watermark. I thought that if we did that well, I was determined to run again this year," McDonald said.