Ronald Reagan's steamroller continued to move ahead in Virginia yesterday as the Republican presidential front-runner picked up 12 more delegates to the party's national nominating convention.

In Northern Virginia's 8th Congressional District, which includes Alexandria, southern Fairfax, Prince William and a portion of Stafford County, three Reagan delegates were elected at a district convention. Reagan delegates outpolled delegates pledged to George Bush by at least a two-to-one margin.

In the 10th Congressional District, which covers Northern Fairfax County, Fairfax City, Arlington, and Loudoun Counties and Falls Church Reagan delegates easily defeated Bush delegates by more than a 2 to 1 margin.

Elsewhere, Reagan also shut out Bush at conventions in Norfolk and Roanoke. The former California governor's supporters predicted yesterday that Reagan would win every one of Virginia's 51 delegates.

Earlier in the month, Reagan had picked up all nine delegates elected in three other district conventions in the state and yesterday's acquisitions brought his total in the state up to 18 of the 30 to be elected at district meeetings. The remaining 21 will be chosen at a statewide convention in Richmond on June 6 and 7.

Guy O. Farley, Reagan's Virginia cordinator was confident that Reagan would be nominated at the Republicans' Detroit convention and would "definitely" beat President Carter in Virginia in the November election.

Bush's main hope for winning delegates in Virginia had been in Northern Virginia and in the Norfolk area. But State Del. James H. Dillard (R-Fairfax), an early Bush supporter, said many of the former CIA director's backers had been disheartened by his losses in recent primaries.

"The way the campaign is going nationwide a lot of people think there's no point to hopping on to something that doesn't look like it's going anywhere," Dillard said at the 8th District convention in Springfield. g

Farley agreed that the turnout at the convention -- about 820 party workers -- was down from four years ago. "There's been quite a lot of apathy during the last month" because of Reagan's strong lead in the delegate race, he said.

Even so, he applauded the Bush supporters for their persistence. "If he (Bush) wasn't in there all we would be hearing about is Kennedy versus Carter," Farley said.

Four years ago Reagan got 37 of Virginia's Republican convention delegates to 14 for former president Ford. Ford later defeated Carter in the state, making Virginia the only state in the south that Carter failed to carry in the 1976 election.

In a spirited contest for 8th District Republican chairman yesterday, Robert K. Cunningham, who has held important positions in a number of major GOP campaigns, defeated Benton K. Partin, former state legislative aide to Robert Thoburn, a strongly conservative candidate in the party's June 10 congressional primary.

Thoburn's opponent in the primary is former representative Stanford Parris, who was more enthusiastically received at the convention than was Thoburn, a former state delegate from Fairfax.

As his supporters shook bells and applauded loudly in a high school gym, Parris declared "You must decide this election on electibility. I believe I'm the candidate with the broadest base to appeal to the voters in the fall."

Thoburn concentrated his attack on incumbent Democratic Rep. Herbert E. Harris, accusing him of helping to weaken the national defense and economy. "Herb Harris has made an in-kind contribution to my campaign," Thoburn said. "He supplied the hot air for our balloons. He has been inflating our money for years. Now it's time to deflate him."

One Reagan supporter at the convention was state Sen. Wiley Mitchell, (R-Alexandria) who earlier had supported Sen. Howard Baker, Mitchell said he switched to Reagan after Baker withdrew from the presidential race because "the nomination of Ronald Reagan is inevitable and the highest priority of the Republican Party now must be the defeat of the incumbent president.