BRATTLEBORO, Vt. – The Vermont Yankee nuclear plant’s owners asked a federal judge Thursday for a preliminary order to keep the plant open while a lengthy legal fight over the state’s effort to shut it down unfolds.

The hearing on the preliminary injunction, which is to continue for a second day today, quickly cut to the meat of the case, as Entergy Corp. lawyer Kathleen Sullivan accused the Vermont Legislature of improperly treading on nuclear safety questions that are the exclusive domain of the federal government.

Lawmakers tried to hide the fact that safety was their real concern, she added, going so far as to change the title of one piece of legislation to take out the word “safety” and replace it with “reliability.” But she urged U.S. District Judge J. Garvan Murtha not to be fooled.

“It is exactly safety, safety, safety that Vermont has sought to regulate,” Sullivan said in court Thursday.

Three lawyers from the Vermont attorney general’s office sought to counter Entergy’s legal attack by pointing to the company’s many statements since it bought the plant in 2002 that it would abide by state rulings on its future. Only now, as the state has come to a decision Entergy doesn’t like, has the company gone back on its word, the state’s lawyers argued.

Assistant Attorney General Bridget Asay urged Murtha to reject the request for an injunction. “It’s our position that Entergy will not succeed on the merits and that this court … should not grant the injunction,” Asay said.

It was not known when the judge might rule on Entergy’s request.

Asay also disputed Sullivan’s contention that the Legislature was focusing on safety, saying lawmakers discussed safety but understood that it was barred from passing laws related to it.

The company maintains that Vermont changed the rules with a 2006 law giving the Legislature a say over a relicensing decision that Entergy understood when it bought the plant would be left to the state’s utility regulators. New Orleans-based Entergy says that change voids the promise it made to take orders from the state. The state Senate voted last year to shut the plant down in 2012; the House hasn’t acted.

But Sullivan went further than Entergy has previously in trying to forestall any justification Vermont might cite in its bid to close its lone reactor when its initial 40-year license expires next March.

The state has cited environmental and economic reasons for shutting down the plant. It’s old and in recent years has leaked radioactive tritium into soil and groundwater. It experienced a spectacular cooling tower collapse in 2007.

Sullivan sought to carve out new ground by arguing that not only is the state barred from regulating the plant on safety, but on economics as well. That’s because Vermont Yankee doesn’t sell power to in-state consumers directly, but rather sells its electricity at wholesale into a power grid serving all of New England. That means economic issues at the plant are overseen not by the state but by the Federal Energy Regulatory Commission, Sullivan said.

Entergy witnesses and lawyers sought to make the case that the clock is ticking on a ruling. John Herron, a senior Entergy executive overseeing operations at 10 nuclear plants, said he needs to decide by late July whether to spend about $65 million to buy new uranium-based nuclear fuel, which would be specially fabricated for Vermont Yankee and installed in the plant during a refueling and maintenance outage scheduled for this fall.

Uncertainty about the plant’s future is clouding that decision and is causing great anxiety among the plant’s more than 600 workers – and even some departures of key senior engineers, Herron said.