The arithmetic of the approaching Virginia Democratic convention now heavily favors Andrew Miller.

According to the state party count, he came out of the April 15 delegate elections with 999 first-ballot votes for the nomination to the U.S. Senate, more committed delegates than the next three candidates combined.

Estimates of his support among about 700 uncommitted delegates, including 116 elected officials, range from 200 to 350. Even the high estimate, however, leaves him about 50 votes short of the majority he needs.

The former Virginia attorney general, who narrowly lost the Democratic nomination for governor last year, again is within reach of a party victory, but is being frustrated by two factors. One is the tenacity of his opponents, five of whom still cling to a hope for the nomination. The other is a distinct lack of fervor among these opponents and their supporters for MIller as a second choice.

The tenacity of the five trailing Miller is easy enough to explain. All have made a heavy investment of time and money in the race that cannot be lightly abandoned. For the least two of them, this is a last fling at statewide politics and they are determined to see it through until the end. Fo r at least two others, the race could build political recognition for future campaigns and they might as well get the most out of it by exposure at the June 9-10 convention in Williamsburg.

The lack of fervor for Miller as a second choice among those favoring other candidates, perhaps a majority of the convention, is more difficult to understand.

In a year in which the Democrats again face formidable Republican opposition, Miller is the party's best known candidate. He takes moderate to conservative positions on issues, well within the range of political orthodoxy in Virginia. He has devoted his adult life to Democratic politics in the state and from 1973 to 1977 was the only statewide officeholder elected as a Democrat.

Why then, when he holds such a large plurality over seven lesser known opponents, is there not a rush, at least in the name of party unity, to make him the nominee?

The answer seems to lie in the ambiguous role Miller has often appeared to play in public office and politics. These ambiguities have left him not quite trusted by either the liberal or conservative wings of his divided party.

Miller, for instance, has always endorsed ratification of the Equal Rights Amendment. However, when he was attorney general, his office appeared at a critical turn in General Assembly consideration of ratification, to give aid and comfort to ERA opponents with a legal memorandum raising the prospect of social horrors - that might flow from the amendment.

Miller dismayed many teachers and Northern Virginia public officials two years ago by lending willing legal support to Gov. Miller Godwin's successful attack on public employe collective bargaining agreements. The teachers and local officials thought they had followed Miller's collective bargaining opinions to the letter.

He sometimes peeved influential lawyer members of the assembly by what they considered to be overconfidence in his positions on legal issues and lack of candor about their out-come.

For instance, he warned House leaders that their reapportionment plan based on the 1970 census would not survive the one-man, one-vote test in the courts. When it did, he irritated them by taking credit for its defense. At the same time, the Senate reapportionment plan, which he said could be easily drfended, was modified by the courts.

In this campaign, Miller upset both conservatives one side and liberals and black Virginians on the other with ambiguous statements on the federal Voting Rights Act, which regulates the elections in Virginia and other states that once enforced racial segregation.

On the occasion of a political endorsement by some black elected officials, he said he favored extension of the act, scheduled to expire in 1982. His staff later insisted that he meant by "extension" that the act should be applied to all the states, regardless of evidence of past discrimination.

In politics, Miller has tried to play a heavy role as a promoter of good Democratic candidates, but has hurt himself by appearing to urge more than one to run for the same office - such as lieutenant governor last year and Congress in the 9th District this year - and thus raising doubts about the sincerity of his support.

He speaks often of unifying the party, but campaigners for Democrats Elmo Zumwalt, Jimmy Carter and Henry Howell perceived him as a restrained ally when theyfought uphill battles against opponents appealing to the conservative instincts of the Virginia electorate.

In this race, Miller has bluntly told the Democrats that he is their only electable candidate, a campaign theme that has grated on the pride of his five opponents who are still alive.

perhaps the least offended by the Miller style has been G. Conoly philips, the Norfolk City Council member and born-again Christian who, nevertheless, believes Miller cannot win the general election because he does not excite the voters.

Many Democrats who believe that Miller will be the nominee also think it likely that Phillips and his conservative supporters, perhaps a majority of them Republican voters in recent elections, will be the ones to break the impasse by swinging to Miller.

If so, it will create another ambiguity in the Miller career. Not quite a year ago, he blamed Republican participation in hthe Democratic primary as a major reason for his loss of the nomination for governor.