The Democrats swept to victory in Arlington last night, winning two seats on the County Board and the sheriff's contest.

In the County Board race final returns showed incumbent board Chairman Albert C. Eisenberg reelected with 29 percent of the vote. His running mate, William T. Newman, won with 26 percent, becoming the first black to serve on the board since Reconstruction.

Republican-backed independents Jane H. Bartlett and Dorothy T. Grotos, who lost bids for the two seats on the five-member board, had 24 percent and 21 percent respectively.

In the hotly contested sheriff's race, incumbent Democrat James A. Gondles Jr. won decisively in what many had expected to be a close race. He had 54 percent of the vote to 46 percent for challenger Ronald B. Hager, a former chief deputy who was a Republican-backed independent.

In the board race, Eisenberg and Newman had campaigned on the theme that things are going well under the Democrats, citing the county's low tax rate and reputation for good services.

Eisenberg said last night that the results prove "people are very satisfied with the government they have in Arlington, which is progressive, compassionate and forward-looking."

Newman said he was "just totally elated." During the campaign he had played down his race and emphasized his civic involvement, such as his membership on the Planning Commission.

Newman said his election proves Arlington voters "are willing to accept a person based on his qualifications."

Some attributed a slightly higher than expected turnout in some precincts to Newman's candidacy. In the Glebe Precinct, which includes the predominantly black Nauck neighborhood, Newman's race was of primary interest, some voters said.

"Newman, that's who brought me out, definitely so," said voter Ruth Young.

Bartlett and Grotos had made political balance their chief issue, arguing that their presence on a Democrat-dominated board would ensure open government.

"It was a tough race. I'm glad I tried," said Bartlett, a civic activist and former member of the Planning Commission.

Grotos, who served on the board form 1976 to 1983, said her showing meant that "people that know and understand what's going on in the county have realized that I am the most honest and hardworking candidate."

The sheriff's contest was marked by great acrimony between two former friends and colleagues.

At its height, four charges of impropriety were under investigation by various county or court authorities. There were accusations that Gondles had purchased for the jail $3,000 in gym clothes from his father-in-law's company, and that Hager had violated state and county laws by purchasing a truck at a sheriff's department auction while he was employed there. Both men denied any impropriety.

Hager, who leveled numerous charges of mismanagement against Gondles, said he had no apologies for the race he ran. "I said everything I wanted to say during the campaign . . . . "

Gondles said, "I feel pretty good. I won. Arlington voters said loudly and clearly 'no' to negative campaigns."