ALBANY – The state Legislature is expected to override all of Governor Pataki’s budget vetoes, as relations between the two branches of government fall to historic lows and the state veers toward a constitutional crisis.

Leading the revolt, the Democratic controlled Assembly yesterday overrode about half of Mr. Pataki’s vetoes, voting to restore more than $1 billion of Medicaid spending that the governor wants to cut. The Senate is expected to follow suit today, which would effectively terminate budget negotiations and set the stage for a historic legal battle over the separation of powers in state government.

With almost no allies to speak of in the Capitol building, Mr. Pataki took his case to the public yesterday, dialing up the editorial boards of several newspapers and speaking directly to editors. Two weeks ago, the lame-duck governor muscled his way back into the budget process by vetoing $2.9 billion in spending and tax cuts and declaring a good portion of his vetoes immune from legislative overrides.

It was an aggressive move that many thought would push lawmakers into a corner. Instead, they declared war with the governor and challenged Mr. Pataki to make the next move. The governor has said he would refuse to enforce budget items that he found unconstitutional.

Such a refusal would likely trigger a court fight. Legislative leaders have said they might sue the governor to force him to spend the money, and interest groups such as the health workers union have also threatened legal action.

Today, the Republican Senate is expected to complete the overrides initiated by the Democrats, sources said. The Senate is also expected to move forward on overrides of vetoes that can only be initiated by the chamber, sources said. Those additional vetoes would derail Mr. Pataki’s efforts to link a tax credit to educational expenses such as tuition and tutoring. The legislative version of the credit doesn’t restrict how the money is spent.

And the overrides would put off debate over lifting the cap on charter schools in the state.

If completed, the overrides would be the first large-scale rejection of the governor’s budget vetoes since 2003.

Bolstered by a recent court decision that solidified his budget powers, Mr. Pataki vetoed large portions of the legislative spending plan – including Medicaid spending and the property tax rebates – on the grounds that they violated the constitution. Mr. Pataki is claiming that lawmakers overstepped its powers by illegally altering the words of his executive budget and is threatening not to enforce those items that he said were in violation.

The budget items whose constitutionality Mr. Pataki denied include $1.3 billion in Medicaid spending and a property tax rebate program costing more than $750 million. Mr. Pataki in his vetoes argued that the Legislature illegally modified his original budget language in both of those cases. Mr. Pataki had wanted a property tax program that would go to homeowners in school districts that adopted a spending cap. The Legislature removed the spending cap, which Mr. Pataki said constituted an illegal language change. The governor also says lawmakers illegally stripped away reforms he wanted for the Medicaid entitlement program.

The Court of Appeals in 2004 ruled that the Legislature could eliminate an appropriations item or reduce it, but it can’t change the measure’s words, which determine how the money is spent.

Mr. Pataki, who is considering running for president, has cast himself as the lone state leader willing to rein in government spending and challenge entrenched interest groups.

The strategy of this plan was to force the Legislature to compromise to avoid a legal battle that would likely hold up the appropriations under question or could toss them out completely.

To the surprise of many here, legislative leaders have apparently decided that they are better off overriding the vetoes than going for a compromise.

The decision wasn’t made easily. Sources said the Senate’s majority leader, Joseph Bruno, asked the speaker of the Assembly, Sheldon Silver, to hold off on the overrides at least until today. Senate Republicans have expressed fear that a war with governor could threaten the survival of the property tax rebate plan that they are counting on to curry favor with voters before the November elections.

The Assembly’s actions narrowed the Senate’s political options, pushing it over the edge, sources said. “We’ve gone to war with the governor, and it’s a foolish thing to do. Nobody wins at the end of the day,” a legislative source said.

During the day, some Senate lawmakers openly advocated continuing negotiations with Mr. Pataki. “We don’t want to just take a macho attitude and say let’s override,” Senator Nick Spano, who represents Westchester County, said. “We need to reduce the rhetoric a bit and sit down at the table.”

Mr. Bruno told reporters yesterday that he was exasperated with the governor but still held out hope for coming to an agreement. “We are practically speaking nowhere,” he said. “I don’t know where the governor has been. His budget director – and the rest of them – they don’t seem to have it together. You can’t get them to speak with one voice.”

Mr. Silver rejected Mr. Bruno’s request and presided over the Assembly’s session yesterday, something he rarely does. He buoyantly announced each successful override with an almost sing-song voice. Assemblymen who spoke during the session described the governor’s actions as blackmail and a threat to democracy.

The governor’s office appears to be searching for allies outside the Capitol. One high-ranking Republican official questioned why Republican candidates for governor, John Faso and William Weld, weren’t backing Mr. Pataki more publicly.

“I have publicly stated I support his vetoes,” Mr. Faso, a former assemblyman, told The New York Sun. “I don’t think you can be a credible candidate for governor and not support Pataki’s position.” Mr. Weld’s campaign did not respond to a request for comment.

The official also said the state comptroller, Alan Hevesi, a Democrat, has been silent during the budget debate and hasn’t taken a position on the size of the out-year gaps produced by the budget. Mr. Pataki has said the gaps would be $6 billion, but lawmakers say that figure is greatly exaggerated.

A spokesman for the comptroller said Mr. Hevesi’s office was working on a comprehensive budget report and would produce extensive data shortly.