In a confusing but enthralling series of moves, the Virginia House of Delegates failed by one vote yesterday to approve a measure to legalize betting on horse races, then decided to reconsider and vote again today.

The voice was 50 to 49 in favor of the bill, but a constitutional majority of 51 (one more than half the House membership) was needed. The net effect is that the fate of the proposal is still in doubt.

Meanwhile, attention has focused on that one vote by which the parimutuel bill failed. Del J. Warren White (D-Norfolk) is the source of to-42 vote Tuesday meant the bill was safe, he left town for Greensboro, N. C., to attend a business meeting that is held only once a year.

"They've offered to send everything from horses to planes to go get him," said his beleaguered assistant, Nettie Trimyer, as she sat in White's office answering the phone (most of the calls were from reporters). Indeed, a few minutes later John Lawrence, an aide to proracing Del. William T. Parker (D-Chesapeake), appeared in the office. Lawrence is a licensed pilot and Parker owns a plane ready to leave for Greensboro, N. C., at a moment's notice.

Ironically, White is opposed to parimutuel betting but was willing to vote for the bill because race tracks could be established only if a referendum next fall is successful.

Equally ironically, White, who had open heart surgery last November, is rarely absent, and until this year had gotten through 16 years in the General Assembly without creating much controversy. This year he has been in the middle of two - the Equal Rights Amendment (which he favors), thanks to his new assignment as chairman of the committee that killed that measure last week, and now horse racing.

Today, after an approximately 20-minute reprise of the arguments against pari-mutuel betting, and the 50-to-49 vote for the bill. House Speaker John Warren Cooke ruled that the 51 vote majority was needed since the measure deals with establishment of a state agency (the racing commission) and the levying of taxes. Cooke voted against the bill.

Hoping to squelch the bill once and for all, opposition floor leader Del. Richard R. G. Hobson (D-Alexandria) moved to have the House reconsider the vote. If the delegates had voted against reconsidering, that would have prevented the supporters from bringing it back to the floor again this session.

The House agreed to reconsider on a 55-to-42 vote - and at that point chief sponsor Del. Raymond R. Guest (R. Warren) jumped to his feet and moved to "pass it by for the day" - giving him a chance to retrieve White at least. (Requests to pass something by for the day are not voted on.)

The outcome - even if White is flown back from North Carolina in time - still is in doubt as several members are known to be wavering. Looking at yesterday's tally, Guest lost three of his 53 votes - White, minority leader Jerry H. Geisler (R-Hillsville) and Joseph P. Crouch (R-Lylchburg).

"I voted for it yesterday because of the referendum aspect," Crouch said. "But after talking to my campaign manager I realized that the fact I'm against pari-mutuel betting would not be understood by my constituents if I voted for this bill."

Crouch said a poll showed that people in his area were heavily against the proposal.

Copies of the vote tally were being passed around yesterday like racing forms, and reporters were jokingly offering to make bets on the eventual outcome. Since Tuesday's preliminary vote, normally an indication of how the measure will fare, was a hand count, nobody could be quite sure who changed his or her mind - except the chief floor leaders and they were not telling.

"I can't tell you the tricks of the trade," Hobson said, "We've just been doing a little missionary work, that's all." Hobson said he had not expected to succeed today, and moved to reconsider because he knew the other side could get someone on the prevailing side to ask for that vote today if he did not. "I thought we should move while the mood was going our way," he said.

Others were calling the move a tactical blunder and noting the possibility that vehemently anti-pari-mutuel Hobson could become the man who saved horse racing in Virginia.

It was clear that some serious lobbying would be going on before the vote tomorrow.

"One man who is undecided even asked me to pray for him," said Del. Robert Thoburn (R-Fairfax), a Presbyterian minister who is against the bill because he believes gambling is immoral. Thoburn made his maiden speech on the House floor yesterday, speaking against the bill, as did Del. Elise Heinz (D-Arlington).

Heinz said the power of the proposed racing commission, which would have five members appointed by the governor, was awesome, and noted that the proposed bill would prevent a person who had had even a minor charge against him from "even selling popcorn at one of our race tracks."

Once again, the voting failed to follow any discernible pattern.

Northern Virginians voting for the bill were Democrats James F. Almand, Floyd C. Bagley, Earl E. Bell, David G. Brickley, Gladys B. Keating, Martin H. Perper, Richard L. Saslaw, Warren G. Stambaugh, Raymond E. Vickery and Republicans Vincent F. Callahan, and Robert E. Harris.

Northern Virginians voting against the bill were Democrats Hobson, Heinz, Mary A. Marshall, Dorothy McDiarmid, Kenneth R. Plum, and Republicans Gary R. Myers and Robert Thoburn.