Former attorney general Andrew P. Miller was on the threshold last night of locking up Virginia's Democratic nomination to the U.S. Senate as a result of party caucuses in the state's 136 counties and cities.

With more than 85 per cent of 2679 delegates accounted for the official Democratic Party headquarters tally gave Miller 899 votes. More complete but unofficial counts by Miller aides and other campaigners aides showed him nearing the 1,000 delegate mark.

The party tabulation gave 412 delegates to Miller's closest rival state Sen. Clive L. DuVal II of Fairfax County. Born-again Christian G. Conoly Phillips, a Norfolk City Council member, was third with 313 delegatesThe party count listed 523 delegates uncommitted.

Athough yesterday's delegate selection process apparently left Miller about 400 votes short of the 1,398 needed to win the nomination at the June 9-10 convention in Williamsburg. his commanding lead over the other candidates makes him a heavy favorite

Miller is expected to win most of the 116 votes allotted to Democratic General Assembly members and members of Congress. The rest of his majority must come from uncommitted delegates and second-choice votes from delegates committed to other-candidates who trailed far behind the leaders yesterday.

Those candidates and their delegates totals, according to the incomplete party tabylation, are: former Fairfax County supervisor Rufus Phillips, 135: former delegate Carrington Williams of Fairfax 81: state Sen. Hunter B. Andrews of Hampton, 65; former Fairfax supervisor Fred Babson, 3, and Falls Church feminist Flora Crater, 2.

Despite the Miller plurality, a spokesman for Duval said. "The mass meeting results demonstrate that Andy Miller failed to put it together. When the smoke clears, we will be right there where we always have been closing in on him and making him sweat."

Proclaiming his campaign to be a "ministry unto the Lord," Conoly Phillips threw a politically unusual element into the day's delegate-selection process by mobilizing hundreds of Christian supporters to attend mass meetings for him across the state.

They were able to win substantial bloes of delegates for him in some populous cities and counties, especially in the Hampton Roads area and the Richmond suburbs. However, Conoly Phillips did not appear to be in a position at day's end to alter the course of the convention.

To deny Miller a majority now would require a determined coalition of virtually all the other candidates, including a fusion of delegates committed to moderate-liberal DuVal and party outsider Conoly Philips.

Campaign managers in the Senate race have generally assessed the Conoly Phillips supporters to be politically conservative. In interviews during the days before the mass meetings, most said they expected the bulk of the Conoly Phillips delegates eventually to wind up in Miller's column.

Conoly Phillips' campaign manager Ed Mabry said in an interview last night that his candidate probably would decide within the next few days whether to campaign for more delegates or to endorse another candidate.

In answer to questions, Mabry said, "Conoly had very warm feelings toward Miller throughout the campaign."

Mabry said his count shows Conoly Phillips with 449 delegates and Miller with about 1,000. "Obviously either DuVal or Conoly could put Miller over if the delegates of one of them went that way." he said Mabry agreed that it would take a concerted effort by all of the candidates to defeat Miller.

Miller operated telephone banks in his strongest areas to turn out supporters to the mass meetings. He ran best in the small cities and counties of the 6th, 7th and 9th congressional districts in western Virginia and won 95 to 155 delegates in Richmond. However. Conoly Phillips, thanks to a coalition with delegates for Rufus Phillips, managed to win 34 delegates to 32 for Miller in Miller's home county of Henrico, a Richmond suburb.

Miller's race for the Senate nomination comes a year after he was defeated narrowly in the Virginia guber-natorial primary by fromer lieutenant governor Henry E. Howell.

Howell for 10 years the champion of the liberal wing of the divided state party, is backing DuVal in the Senate race.

Three Republican candidates are locked in what appears to be a wideopen race for the GOP nomination. They are former Gov. Linwood Holton, former Navy secretary John Warner and former national Republican cochairman Richard Obenshain. The Senate seat they seek is being vacated by the retirement of RepublicanWilliam I. Scott.