A defiant William Weld yesterday stood by a tax-cutting proposal that budget analysts are describing as bizarre and that one of his key conservative supporters says “hasn’t been well thought through.”

The Republican gubernatorial candidate said on Tuesday that he wants to abolish the state personal income tax for New Yorkers earning $75,000 or less.

While the proposal has earned praise from Governor Pataki, the plan is being ridiculed by fiscal experts, who said it would create an imbalance in the tax system and an incentive against marriage.

A spokeswoman for Mr. Weld’s campaign, Andrea Tantaros, said the former governor of Massachusetts was sticking with the plan and defended it as good policy aimed at young and middle-class New Yorkers.

She said Mr. Weld’s proposal, which would not extend any benefit to married couples whose combined income exceeds $75,000, was not a marriage penalty because those who are currently married would not be paying any more taxes. Ms. Tantaros also said what Mr. Weld was proposing was just one part of the candidate’s broader tax-cutting plan.

Fiscal observers said they were baffled by Mr. Weld’s plan, which is not known to exist in any state. Some political analysts suspected Mr. Weld, who is trailing in the polls to Democratic candidate Eliot Spitzer, is trying to appeal to lower-income New Yorkers who might be reluctant to vote for a Republican.

The executive director of the Fiscal Policy Institute, Frank Mauro, said he couldn’t imagine the state Legislature shifting the entire income tax burden to the minority of New Yorkers who earn more than $75,000. “No state has a cliff like this,” he said. “It would be completely illogical. You wouldn’t treat people in such a perverse way. In a civilized society it would never be tolerated.”

Others said the plan would cause volatility in the tax system by sharply reducing the tax base, forcing the state to depend upon its tax revenue from a much smaller number of people.

The plan was also criticized by a former candidate for governor, Herbert London, a conservative intellectual who has endorsed Mr. Weld for governor.

“The marriage penalty and the cliff obviously suggest that this hasn’t been well thought through,” Mr. London, the president of the Hudson Institute think tank, said. “It needs a lot of tinkering.” He praised Mr. Weld, however, for taking a position that is “bold and unusual,” which he said goes against the Republican grain.

Though standing by his proposal, Mr. Weld’s campaign revised how it would intend to pay for the tax cut after budget analysts and one of his Republican rivals accused Mr. Weld of relying on inaccurate figures.

Mr. Weld said the state could recoup most of the money to support the tax elimination by reducing Medicaid spending to a per capital level of twice the national average. The state spends more on Medicaid than any state in the nation. While Mr. Weld said the state would save $5.3 billion through such a cut to the entitlement program, critics said the actual amount of savings would be at the most half that amount. The $5.3 billion figure is the total local, state, and federal savings, analysts said.

“He has made a major error in making that comparison because the numbers are just wrong,” a Republican opponent of Mr. Weld, John Faso, a former Republican leader in the State assembly, said.

Yesterday, Mr. Weld’s campaign issued a document showing other ways in which the state could make up for the lost tax revenue. For instance, the campaign said, Mr. Weld’s “taxpayer bill of rights” plan, which would restrict the amount of state tax revenue that lawmakers can spend to the prior year’s revenue total, multiplied by the inflation rate plus annual population growth, would have saved New York $23 billion from 1995 to 2005.

After Mr. Weld announced the plan, whose idea was borrowed from an editorial that ran in a newspaper in Binghamton, there appeared to be confusion inside his campaign about what he had actually proposed.

Mr. Weld, at first, indicated that he was proposing to exempt the first $75,000 of income from the New York State income tax. That would mean that a person earning $77,000 would only pay a tax on $2,000 of the income.

He said the plan, which would benefit anybody who pays state income taxes, would reduce the tax liability of New Yorkers by $6.9 billion. Budget analysts, however, predicted that such a universal exemption plan would cost billions of more dollars than $6.9 billion.

When pressed for more details on Tuesday, a spokeswoman for Mr. Weld’s campaign, Ms. Tantaros, appeared to revise what Mr. Weld had said.

She said only New Yorkers who make $75,000 or less would be eligible for the cut and those earning more would be excluded. In other words, a single person earning $75,000 wouldn’t pay any state income tax, but a single person earning $75,001, and taking the standard deduction, would pay $4,226.82.

Ms. Tantaros denies that there was any confusion about the plan.