When Adelard L. Brault is in Richmond, he commands a well-appointed, paneled office overlooking the State Capitol. When the short, peppery state senator takes to the floor of the General Assembly, he gets attention.

After 14 years in the legislature, Fairfax lawyer "Abe" Brault has become one of the powers of the assembly. He is the Senate's Democratic majority leader and next January will become dean of the Northern Virginia delegation.

That is unless a 22-year-old college student, John M. Thoburn, the region's most conservative legislator, is giving Brault, aged 70, the most serious challenge he has known since his first election.

That time Brault won a recount victory by a 40-vote margin over William L. Scott, then a little-known Fairfax lawyer who went on to win three successive terms in the U. S. House of Representatives and a six year term in the U. S. Senate.

But this year Brault is being trailed by the young, dogged Thoburn in a race that has become increasingly bitter.

"He says the same things everywhere, the same distortions and half-truths," Brault complained the other day. "But the issue here is really who's going to be more effective for Northern Virginia."

Brault said he just cannot believe that voters in the 34th Senate District, representing Fairfax City and the east-central part of the county, will choose a "college student over the majority leader of the senate."

His legislative seniority and powerful committee assignments, Brault argues, have meant improved transportation, schools and services for the handicapped in the Washington suburbs. Those improvements will continue if he stays on the job, he said.

But Thoburn, whose mustache and stern composure makes him look older than the junior economics major he is, calls Brault a politician "who has promised one thing and done another."

Four years ago, eight years ago and 12 years ago, according to Thoburn, Brault "ran on a platform to repeal the sales tax on food and nonprescription drugs. Yet, he voted last year to increase the sales tax on food by 25 percent."

Thoburn, who scored an upset primary victory last June over former secretary of the commonwealth Cynthia Newman, a state official, is quick to add that his father's vote on the House floor helped to kill that legislation.

What Thoburn does not say, Brault angrily tells voters, is that the measure in question was a proposal for funding Metro's operations in Virginia without increasing the tax burden on property owners.

"And every responsible member of the Northern Virginia delegation voted for it except two, and one of them was my opponent's father -- who is considered to be on the far right, " said Brault.

The veteran Fairfax lawmaker chides his young challenger for his lack of experience, predicting that his status as a newcomer and his ultra-conservative viewpoints would make him "totally ineffective" in Richmond.

Thoburn counters, however, that Brault has used his seniortiy to support tax increases. "I'm going to Richmond to vote against increasing taxes, and it doesn't take 14 years of experience to do that," Thoburn said.

Citing his own efforts, Brault said he was part of a study group that pushed through legislation that will repeal Virginia's inheritance tax at the end of this year. He sponsored and still supports a bill that would allow state government to grow no faster than the state's economy.

Brault, a lawyer, chairs the General Assembly Laws Committee in the senate and has seniority on four other key legislative panels. in addition, his return to the assembly will put him in the thick of an expected legislative reapportionment battle in 1981.

"I've been labeled a liberal by some and a conservative by others, and, as usual, the true lies somewhere in between," said Brault.

Thoburn, who graduated from his father's Fairfax Christian School and served as his legislative aide in Richmond last session, pledges to work for repeal of the sales tax on food. He suggests that localities use existing revenue sources, if necessary, to pay for Metro.

Despite the constant mention of his father in his campaign remarks, the younger Thoburn denies that his father pushed him to challenge Brault.

"We have a working partnership," he concedes. "But people often think my dad got me to run when he says that I was the one who first go him to enter politics."