ALBANY – Governor Pataki is often mentioned in the past tense in the state capital. Lawmakers can be heard musing about the make-up of a Spitzer administration, and when the lame-duck governor fell ill in February and was hospitalized for nearly three weeks, legislators said they hardly noticed a difference in the Capitol building.

So, it came as a surprise this week when Mr. Pataki uncorked a late-round uppercut that has shaken the confidence of lawmakers who thought they had the upper hand in budget negotiations.

Empowered by a 2004 Court of Appeals ruling upholding his executive prerogatives, Mr. Pataki is expected today to pronounce major pieces of the legislative budget unconstitutional and sweep them away.

To the alarm of lawmakers, he’s taking aim at some of their most cherished spending items, like the Senate’s program to give millions of New Yorkers property tax rebate checks. He’s also using the court’s decision to confront lawmakers on Medicaid spending, which he says is too high. And he’s vetoing the Legislature’s entire revenue bill because it doesn’t include a pro-school choice measure that would give tuition tax credits to low-income parents.

Months before he retires from office and possibly embarks on a presidential campaign, Mr. Pataki is flexing his muscles and seeking to reclaim the mantle of fiscal conservatism. It’s a label that many of his critics stripped away from him long ago after watching state spending more than triple since he took office in 1995.

“He’s looking to make a statement,” a political scientist in Albany and publisher of the Legislative Gazette, Alan Chartock, said. “What he wants to do is go to a red state and say, ‘Look, I’m a fiscal conservative.'”

Legislative leaders yesterday denounced the governor’s tactic and said they have no intention of yielding to the governor’s budget demands. They also expressed exasperation about their options and signaled that they prefer to negotiate rather than go to court.

“Is there room to negotiate?” the Republican Senate majority leader, Joseph Bruno, said. “Absolutely. We want to get a result.” He said it would be “very unfortunate and tragic” if the governor struck down the Legislature’s property tax rebate program.

Senator Dean Skelos, a Republican of Nassau County, said of the governor, “It’s not just for him to decide what spending should go out the door or not.”

Mr. Pataki wanted to give $400 school tax rebate to homeowners living in school districts that adopt a spending cap. Lawmakers opted for a statewide rebate program that did not insist on such a cap. Mr. Pataki insists that lifting the spending cap would erase the benefit of the tax break by allowing school districts to raise their taxes.

The Democratic speaker of the Assembly, Sheldon Silver, accused the governor of trying to “hide behind technicalities,” and, noting the governor’s presidential ambitions, compared him unfavorably to Mitt Romney, who is also considering a White House bid. But the speaker also deflected questions about whether he would sue the governor, saying that New York residents may choose to sue instead.

“If we override it, the next step is his,” he said. “I’m dealing with substance. I’m not going to be the governor of technicality.”

Mr. Pataki’s legal threat, which some are calling the “nuclear option,” focuses the most pressure on Mr. Silver, who has vowed not to support the governor’s tuition tax credit plan. Senate Republicans, who are looking to protect the property tax rebate, could block an override on the revenue bill and try to force Mr. Silver to approve a tax credit plan that the governor wants. If they do that, they may get the governor to relax his opposition on the property tax rebate program.

Lawmakers can override vetoes on items that Mr. Pataki believes are unconstitutional, but the governor’s administration is expected to argue that his disapproval isn’t subject to an override. They can sue him, but they would be going against the grain of a raft of court decisions delineating executive budget powers that have sided with the governor.

“We’ve been through this time and again,” Mr. Pataki said yesterday at a press conference. “We’ve won every single lawsuit.”

If they don’t sue, however, lawmakers could be trapped in the future by the precedent set by the governor. “This governor has pushed the envelope to the breaking point in terms of the power of the executive,” the legislative director of the New York Public Interest Research Group, Blair Horner, said. “If he succeeds, it could hamstring legislatures in the future.”