Governor Spitzer, despite his stern assurances to the contrary, will have no choice but to yield to political and budgetary pressures in coming weeks and consent to a 12% income tax increase on the state’s top earners, according to Assembly Democrats.

In a direct challenge to Mr. Spitzer’s line-in-the-sand pledge not to raise taxes, Assembly Democrats are poised to pass legislation that would lift the income tax rate of residents making more than $1 million a year to 7.7% from 6.85%.

The combined local and state income tax rate for millionaire residents of New York City would rise to 11.35% under the bill. By comparison, top earners in Philadelphia, Connecticut, and New Jersey pay 7.57%, 5.0%, and 8.97%, respectively.

The rate increase advanced by the Assembly would be authorized for five years and would generate an estimated $1.5 billion in state revenue a year, lawmakers say. In later years, the state would be required to dedicate the money to mass transit projects and road and bridge repairs.

As a candidate and as governor, Mr. Spitzer has vowed not to increase taxes in New York, one of the highest-taxed states in the nation. He reiterated his stance this week in responding to reports about the Assembly plan. His opposition was shared by the Republican majority leader of the Senate, Joseph Bruno, who told reporters he wasn’t “sure it’s such a good idea.”

Assembly Democrats, however, say they are growing confident that when the dust settles on budget negotiations this month, Mr. Bruno and the governor will have come around. The money that the tax increase would generate, they say, is too attractive to turn down, especially at a time when the state’s revenue projections have plummeted and spending obligations have grown.

In the past two months, the state has lowered revenue estimates by more than $600 million. Meanwhile, the Spitzer administration is searching for a way to close a more than $9 billion gap in the Metropolitan Transportation Authority’s five-year capital plan.

Democrats say the so-called Millionaire Tax is a more equitable and dependable way to replenish state coffers than the alternatives offered by the Spitzer administration, which is proposing to balance the budget by relying heavily on a variety of fee increases and closures of “tax loopholes.”

Democratic lawmakers say Mr. Spitzer’s ability to block their plan hinges on the Senate sticking to its position. If both houses were to unite, Mr. Spitzer may not be able to stop the hike without risking a late budget, which is due April 1, or a legislative override.

While Mr. Bruno says he’s against raising taxes on the wealthy, his conference has been more aggressively opposed to the fee hikes and loophole closures. Yesterday, Senate Republicans attacked a provision in the governor’s budget that would raise Motor Vehicle Insurance fees by $15.

Mr. Bruno has also criticized the governor for trimming the growth of a property tax relief program and for not doling out more money for Long Island public schools.

With money from the Assembly’s plan, the Senate could conceivably restore the governor’s cuts and have enough left over to eliminate the governor’s revenue actions. Such an option may be too irresistible to pass up, Democrats say.

Mr. Spitzer “will never be in favor of it as long as the Senate is opposed to it, but at some point, $1.5 billion is real money,” a high-level Democratic assemblyman said. “Joe and he are never going to acknowledge it, but you just watch. Let’s see what happens: It’s not going to happen. It’s not going to happen. It’s not going to happen. It happens. That’s the scenario we could get.”

The last time state lawmakers approved a major tax hike was in 2003 — at the same time federal tax cuts were enacted — when they overrode Governor Pataki’s veto and raised the rate for residents earning more than $150,000 to 7.5% and for residents with income of more than $500,000 to 7.7%. The tax increase was phased out over three years.

New York’s top income tax rate is in the middle range among states. Ten states have a top rate of more than 7.7%, but they apply the rate to brackets much lower than $1 million. The Assembly’s plan would come closest to approximating New Jersey’s structure, which imposes an 8.97% tax on residents earning more than $500,000.

According to 2004 state tax department data, about 47,000 tax filers, including 20,000 nonresidents of New York, reported adjusted gross income above $1 million. Representing less than 1% of all filers, they paid $7.96 billion in state income taxes out of the $26.1 billion collected by the state, about 30% of state income tax revenue.

By breaking his tax pledge, Mr. Spitzer would be sure to encounter an outpouring of anger from business groups and fiscal conservatives, who argue that raising taxes would only drive away more employers and residents.

Mr. Spitzer, on the other hand, would also help to shore up his liberal base. Leaders of the labor-backed Working Families Party, an influential advocacy group in New York City, have said in interviews that they would consider withholding their endorsement of the governor when he runs for re-election in 2010 if he refused to support the hike.

Assembly Democrats said Mr. Spitzer’s position clashes with his professed support for progressive policies.

“He’s willing to raise insurance fees on the middle class, and he’s willing to put congestion fees on the middle class … but he won’t tax people who make over $1 million,” an assemblyman of Westchester, Richard Brodsky, said. “It makes no sense on the merits. And it makes no sense on the politics.”

A top aide to the governor said Mr. Spitzer would stand firm. ” He’s not going to break the pledge. He’s been very clear about this,” a senior adviser, Lloyd Constantine, said. “At this point, the governor has determined that the enactment of new taxes is counter-productive to the overall economic goals of the administration. … This is unequivocally a new tax.”