The Virginia Racing Commission rejected two rival plans today to build horse tracks in Prince William County, effectively killing the troubled industry's hopes of bringing racing to populous, affluent Northern Virginia.

The five-member panel, which must issue any track license, said neither Colonial Downs Inc. nor Middleburg businessman James J. Wilson proved that their $20 million ventures could survive in a risky racing market that is struggling for patrons.

The threat of new competition also was a factor in the decision. Commissioners said one of the two proposals--the track planned for Nokesville in western Prince William--would have killed off the state's only existing racecourse, an ailing track operated by Colonial Downs in New Kent County, east of Richmond.

"We all know there has to be some kind of presence in Northern Virginia for racing to work," said Robin Williams, commission chairman. But "putting a nail in the coffin of Colonial Downs was not going to help racing."

The track proposed for Nokesville and another that Colonial Downs wanted to build in Dumfries off Interstate 95 were the industry's first attempts at expansion since the opening of the New Kent facility, which is in financial trouble.

Both plans needed local as well as state approval. Last week, the Dumfries Town Council rejected the Colonial Downs proposal, heeding grass-roots opponents who denounced the gambling the project would bring as immoral and addictive. Prince William supervisors had not taken up the Nokesville proposal, but several had said they opposed it.

Yesterday's decision appears to put the state's fledgling racing industry on life support, 11 years after Virginia voters approved betting on horse races. Racing, in a state slow to embrace gambling, was hailed at the time as the salvation for the state's slumping equine industry.

Colonial Downs executives said yesterday that they've put the New Kent track up for sale, after suffering losses of $6.5 million over two years. Owner Jeffrey Jacobs warned that the state's first venture into horse racing could fail by next spring if a new owner isn't found or finances don't improve.

Wilson called yesterday's vote against him a "political decision to protect Colonial Downs at all costs," and said he does not plan to propose another track in Virginia. "The political reality is, there will never be approval for another track in Northern Virginia."

The industry had pinned its hopes on tapping the potentially lucrative Washington suburbs. But racing is troubled across the nation, as gamblers have given their dollars to lotteries and slot machines instead of horse tracks.

Both applicants planned to rely heavily on off-track betting parlors--where races in other states are simulcast onto television screens in the grandstand--for revenue to sustain live racing.

But the industry's heavy reliance on off-track betting has stirred fierce political opposition in Northern Virginia.

And the industry's declining fortunes could bring calls from an increasingly conservative General Assembly to repeal the 1988 law allowing horse racing.

"The looming question is whether some of us may not want to continue down this path," said state Sen. Kenneth W. Stolle (R-Virginia Beach), who has supported racing but has criticized Colonial Downs. He added that "many of my colleagues believe that gambling . . . is the wrong way to go."

Racing officials said they remain optimistic that the industry can survive in Virginia.

"It's very difficult to open a new track in a new market," Williams said, "but there's a vast undeveloped market for racing that's still out there."