Supporters of a measure to curb the spiraling costs of five nuclear plants under construction in Washington were euphoric this week after voters overwhelmingly approved an initiative giving voters a say in the cost of new energy projects.

But the fight isn't over yet, and the eventual winner is likely to emerge from a courtroom.

Initiative 394 passed by almost a 100,000-vote margin in Tuesday's election, despite a megabucks campaign by its opponents, who promise a legal challenge as soon as it takes effect in July.

Steve Zemke, a leader of the "Don't Bankrupt Washington Committee," called the vote a mandate. "You do not get outspent seven to one and still win unless the voters wanted to send a message," he said.

The initiative requires public votes on any big energy projects, and includes votes on additional bond sales for nuclear plants being built by the Washington Public Power Supply System (WPPSS, pronounced "Whoops"), the construction branch of the state's public utilities.

The measure was drawn up after the construction costs of the five WPPSS nuclear plants ballooned from $4.1 billion to nearly $23.8 billion. The projects have been plagued with cost overruns and delays as WPPSS directors have gone again and again to New York brokers to sell bonds to finance the construction of the plants.

Gov. John Spellman said he was disappointed but not surprised by the measure's passage.

"I think it was a very legitimate, visceral reaction to the fact that WPPSS has spent a lot more money than can be justified and they've obviously suffered a great deal of bad management," Spellman said.

Last month, the state's bond rating was lowered from AA-plus to AA, partly because of the uncertainty about the WPPSS plants. Two of the five plants were "mothballed" last week until 1983 because of the constantly rising costs to complete construction. The campaign against Initiative 394 spent $1.2 million, making it the most expensive campaign in the state's history.

The anti-394 campaign, financed by contractors, construction unions, Wall Street brokerage houses and private utilities, charged that passage of the issue would mean there would have to be six to eight statewide elections a year and would prevent building any new power plants, whether coal, hydro or nuclear.

Opponents also charged the initiative would kill WPPSS plants one, two and three, making future bond sales all but impossible.

WPPSS managing director Bob Ferguson said election night that WPPSS financial advisers have advised him to proceed with the projects as originally planned.

However, Nick Cain, president of the WPPSS board of directors, said passage of the measure is going to make future financing more difficult.