Contenders for the Democratic nomination to the U.S. Senate in Virginia moved yesterday to capitalize on the choice of conservative Richard D. Obenshain as the Republican nominee by painting him as a right-winger and claiming that his selection helps open up the Democratic race.

Obenshain has long been viewed by Virginia Democrats as the weakest of their three likely Senate opponents, and moderates among the eight Democratic candidates, especially those from Northern Virginia, now see a chance to draw a favorable contrast between themselves and the GOP nominee.

"I think they picked their most vulnerable candidate," state Sen. Clive L. DuVal II of McLean said in an interview. "He's a negative kind of politician who takes the 'let's go backward' approach.' His nomination is helpful to me and to several of the moderates in the race, especially the Northern Virginians. Obenshain is not strong up here and we can make a case that we can beat him badly in Northern Virginia."

DuVal is running second to former attorney general Andrew P. Miller in commited delegates, with the Democratic nominating convention in Williamsburg less than a week away. Miller, viewed as a moderate-conservative in his party, declined to be interviewed yesterday but issued a statement making it clear that he intends, if nominated, to run a campaign portraying Obenshain as a right-winger.

"In choosing Dick Obenshain as their candidate for the Senate, Virginia's Republicans have left no doubt that theirs is a party that cares neither for the needs of people or the necessities of progress," he said. "It will be clear when the fall campaign begins that the Republican candidate is a candidate whose philosophy can best be described as standstill and stonewall."

Obenshain's recent, frequent descriptions of Miller as "liberal" have been reminiscent of the successful campaign of Republican Sen. William L. Scott six years ago. Scott unseated moderate Democrat William B. pong by portraying him as a liberal advocate of big federal government and busing of school children to achieve racial balance.

At a press conference last week, Miller said of Obenshain's "Liberal" label for him. "We're not going to let him get away with that this year. We're ready to play political hardball this time."

Yesterday's statement by Miller promised that he would make an aggressive attack on Obenshain. "It will be the responsibility of our party's nominee to make certain others know who the Republican nominee is and what he stands for. This is the key to victory," Miller said.

The Republican nominating session on Saturday, the largest state political convention ever held in the nation, immediately became a factor in the coming Democratic convention in ways that go beyond the choice of the GOP nominee.

Despite a vigorous contest among three major candidates and a dark horse, the Republicans reeled through six roll calls of about 7,500 delegates without divisive fights and without the restructive rules that are, for the moment, governing the Democratic convention.

Miller's seven opponents want the convention to adopt permanent rules that would eliminate temporary provisions that would force trailing candidates out of the race in early roll calls and permit the winner to be nominated only by a majority of those voting rather than a majority of the 2,797 authorized convention votes.

The temporary rules are considered helpful to a front-runner and Miller resisted any change in them until last week when he offered to negotiate with his opponents. All eight candidates will meet with party chairman Joseph T. Fitzpatrick today to try to reach an agreement, and the conspicuous harmony of the Republican convention is certain to be in their minds.

Virginia is the only state in the nation that has failed to elect a Democratic governor or senator in the last 10 years, and party leaders put much of the blame on divisive nomination contests. "Whoever the nominee is this time," DuVal said yesterday. "I think he will want all the losing candidates to go home feeling that they had a fair shot at the convention."

The most puzzling aspect of the Republican convention to Democrats was its failure to nominate former Gov. Linwood Holton. All the Democratic candidates have said in interviews that Holton would have been the toughest Republican foe and many of their campaign staffers have been frank to say he could not be beaten.

Holton, a 54-year-old McLean lawyer who in 1969 was elected the first Republican governor of Virginia in this century, not only did not win but withdrew after the third ballot when he faded to a poor third behind Obenshain and former Navy secretary John Warner.

The moderate Holton tried to sell his electability to prospective delegates with the help of a mostly volunteer staff. But over the long winter and spring campaign, he was no match for his two principal foes.

Obenshain built his winning campaign on the loyalties to him of conservative city and county party leaders. Warner swept up moderates, conservatives and many necomers to GOP politics with a lavish campaign financed by his personal fortune and amplified by the glamor of his wife, Elizabeth Taylor.

Some Holton admirers were dismayed by his withdrawal after three ballots, believing he should have held out for a break in support for Warner and the fourth candidate, state Sen. Nathan H. Miller of Rockingham County.

But Holton campaign chairman Vincent F. Callahan, a state delegate from Fairfax County, said the trend against his candidate set in motion ineversibly by Holton's failure to finish second on the first ballot. "Had we been ahead of Warner on the first ballot, it would have been a different story." he said.

When Holton withdrew, only Miller was left to hold the balance of votes needed by the front-runners to obtain a majority of 1,541. He held on for two roll calls, and then withdrew, a move that assured the nomination for Obenshain.

One of the striking aspects of the Republican convention was the role played by former governor Mills E. Godwin. The last leader of the politically dominant and conservative Democratic party in Virginia before a take-over by the liberal wing of the party in 1972, Godwin became a Republican in 1973.

He is the only person ever elected governor of Virginia twice, once as a Democrat and once as a Republican. He was not expected to play a heavy role in party politics after his announced retirement from elective politics this year and took no part in the GOP convention campaign beyond be coming a delegate from Suffolk.

Then, three days before the convention, he endorsed Obenshain and on Saturday delivered his nominating speech. Obenshain's opponents centered that the Godwin endorsement was not an important factor, but his presence at the center of the successful Republican campaign was suggestive of his continuing influence.