Andrew P. Miller is telling Virginia Democrats these days that their failure to nominate him as their candidate for the U.S. Senate this spring would lead to another defeat at the hands of the Republicans and probably to the end of the state party as a "viable force" in the foreseeable future.

It is a hard sell, a calculated pitch that is meant to jar potential convention delegates into believing, as Miller does, that he is the only Democrat who has a chance of defeating formidable GOP opposition in the fall and thus end more than 10 years of Democratic losses in races for U.S. senator and governor.

"If we don't get the nomination," he said, ". . . I'll be supporting the person (who does). But realistically, looking at the track record, the fact that I am the only candidate who has held state office, the fact that none of the others has the name identification that I developed . . . as attorney general . . . to think that the Democratic party can turn it around would be based more on illusion than practical assessment of the situation."

Miller resigned after seven years as attorney general in 1977 to run for governor. He made electability a major issue in his primary race against former Lt. Gov. Henry Howell, but lost the party nomination narrowly. A month after populist Howell was decisively beaten by Republican John N. Dalton, Miller began his public warnings of unrelieved disaster if the Democrats don't turn back to moderate conservative candidates.

At a state party meeting, Miller said in Howell's presence, "If we nominate a turkey next year, then you can count on getting plucked in the fall."

"The outcome of the general election last fall," Miller said in an interview after a Charlottesville speech, "showed that if the Democratic Party is going to win again, it needs a candidate like Sarge Reynolds, Chuck Robb or Andy Miller."

The late J. Sargeant Reynolds was elected lieutenant govenor in 1969 and died in office two years later. Charles S. Robb was elected lieutenant governor last fall. Miller sees himself, Reynolds, Robb and former U.S. Sen. William B. Spong, defeated by Republican William L. Scott in 1972, as the modern standard bearers of a moderate Democratic Party in Virginia. Scott's retirement is vacating the Senate seat Miller seeks.

"Now the decision is whether the party will choose a candidate of the Spong, Robb, Reynolds, Miller type, the type of candidate with whom there is a real chance of having an influence on public policy," Miller said in the interview.

"If the party realizes that, and we turn the situation around, then Chuck Robb's chances (for governor) in 1981 will look much brighter than they are now. "If we don't turn it around, it will be tough."

Miller not only sees his nomination as a key to Robb's success but to the continued strength of the party in general.

"In terms of the general election," he said, "there is no question but my candidacy is the strongest for the democratic Party. There is a question of whether the party is to be a viable statewide force. If we don't put it together this year, then I believe our long dry spell will turn into a very serious drought."

Miller is not the only candidate for the Democratic nomination who is basing his campaign on an appeal to the party to return to moderate conservative candidates, but he is the best known. The others are state Sen. Hunter B. Andrews of Hampton, former Del. Carrington Williams of Fairfax County and former Fairfax Supervisor Fred Babson.

State Sen. Clive L. DuVal II of McLean is the leading candidate on the more liberal side of the party, although DuVal rejects the liberal label as "unfair" to him. Former Fairfax Supervisor Rufus Philips is running a vigorous campaign as a newcomer to statewide politics.

Also in the race are feminist Flora Crater of Falls Church and Norfolk City Council member G. Conoly Phillips, who is running as a Christian candidate.

Miller's vision of himself as the only viable Democratic candidate naturally rankles some of his opponents. It confirms their opinion, formed during Miller's tenure as attorney general, that he is both exceptionally competent and exceptionally self centered.

One of his leading opponents for the nomination said "There is no doubt that Andy is competent. The trouble is he thinks he is the only one around with any competence."

Miller, who is 45, perceives DuVal, who is 65, as his principal foe and has at times made a point of his own age in the campaign. At a recent press conference in which he stressed the importance to Virginia of electing a senator young enough to serve eeveral terms, Miller was asked whether he was suggesting that DuVal and other opponents are too old to run.

"I don't know the ages of the other candidates," he answered. It was an episode that did not enhance what Miller often refers to as the "camaraderie" among the Democratic contestants.

Losing the nomination last year to Howell after spending a record $1.2 million in the primary campaign was a severe blow to Miller and has turned off some former supporters who now question his ability to win.

Miller himself blames his defeat on three things: (1) published polls that showed him far ahead and lulled supporters into not voting; (2) Republicans who voted for Howell in the belief he would be a weaker opponent for Dalton: and (3) the recent tendency of conservative Democrats to consider themselves independents and stay out of Democratic primaries.

Miller's perception that DuVal is his chief threat this year is corroborated by many party officials and campaign staff members who say that they are the cofavorites for the nomination at the June 9-10 convention in Williamsburg.

However, there is a growing feeling that a large percentage of the 2,795 delegates selected at 136 city and county Democratic committee members in the state. thout committing themselves to any candidate on the first ballot. If this happens, experienced campaign officials say, other candidates, especially Andrews and Williams, will be in a position to split the first ballot more evenly by focusing on the uncommitted delegates.

Miller's main strength is thought to be his close association with party and elected officials during his years as a statewide office holder and political figure. There are 6,700 city and counry Democratic committee members in the state. Miller believes they will make up a majority of convention delegates but possibly only one-fourth of those attending the crucial April 15 caucuses.

DuVal's main strength, apart from his own well financed and staffed campaign, is thought to be the possibility that conservatives will stay out of the convention as they did the 1977 primary and leave it to more liberal party members.

DuVal is supported by Howell and is counting on many of Howell's followers, especially among black Democrats, to stick with him to the end in Williamsburg.

However, the AFL-CIO political action committee, which endorsed Howell last year, has decided to remain officially neutral on the Senate nomination. Moreover Miller seems likely to pick off significant black support. He is expected to receive the endorsement of a group of black elected officials tomorrow at the Richmond city hall.

Miller clearly has the support of most leading party regulars. Six of 10 congressional district chairmen publicly endorsed him last week. His opponents discounted the event because, in the words of one, "those people have always been with Andy." Nonetheless, victory starved, they are with him again this year.

To supplement the corps of party regulars backing him, Miller is canvassing the state for delegates with his opponents. He is making extensive use of phone banks to identify potential supporters and plans to use the phone operation to turn them out to the mass meetings.

Debate over issues is not dominating the delegate selection process and on the issues Miller's positions are not, taken as a whole, strikingly different from those of other moderates in the race.

He says he would have voted for the first, amended Panama Canal Treaty and favors significant amendment of the second. He would transfer Medicare and disability benefits from the social security fund to the general fund. He believes the $61 billion federal deficit should be reduced about 10 per cent through reductions of $2 billion each in foreign aid and anti-recession programs and a 1.2 percent agerage reduction in all other federal programs. He would not vote to expel Virginia's senior U.S. senator, independent Harry F. Byrd Jr., from membership in the Democratic caucus.