The nonpartisan Tax Policy Center, after considering conservative critiques of its recent, much-publicized analysis of Mitt Romney’s tax agenda, said on Thursday that its conclusion stands: His proposals would mean big tax cuts for the highest-income taxpayers and increases for everyone else.

“A reform proposal that meets the five goals” that Mr. Romney has outlined for overhauling the tax code “would have to raise burdens on middle-class households,” the three authors of the analysis wrote in a nine-page response to the criticism.

That is inevitable, they said, mainly as a consequence of two of Mr. Romney’s five goals: His insistence that any tax overhaul should be revenue-neutral, neither reducing nor increasing annual budget deficits, and that it should protect or even sweeten existing tax breaks for savings and investment like those for dividends and capital gains income.

If Mr. Romney were willing to either increase deficits or reduce the tax breaks for investors, the authors said, then taxes would not necessarily have to go up for lower- and middle-income Americans.

As his other three goals, Mr. Romney would cut income tax rates by 20 percent, eliminate the estate tax and end the alternative minimum tax, which was designed to ensure that the richest taxpayers did not escape liability due to various tax breaks but which increasingly threatens moderate-income households.

Some critics of the original study by the Tax Policy Center, which includes former economic advisers to presidents of both parties, have said that Mr. Romney has not proposed a detailed enough plan to analyze so conclusively.

But the authors responded, “It is still possible to examine the broad implications of the five goals,” using information from Mr. Romney and his campaign advisers.

As in their original analysis, the authors described how they essentially gave Mr. Romney the political benefit of the doubt, assuming that to keep his plans from adding to annual deficits he first would end all tax breaks for the wealthiest Americans – like those for mortgages, health insurance and charitable contributions (though not for investments and savings) – before touching the same tax breaks for lower-income Americans.

The authors also rebutted criticisms that their initial report did not sufficiently account for the potential benefits of Mr. Romney’s separate proposals to lower the corporate tax rate to 25 percent from 35 percent, reduce other business taxes and cut domestic spending deeply.

The authors said they initially had assumed that Mr. Romney would offset the cost of his proposed cut in the corporate tax rate by scaling back other business tax breaks, but Romney advisers said that would not be the case. Yet that would mean “an even larger tax cut on high-income individuals, requiring even larger cuts to tax expenditures” – that is, tax breaks like those for mortgage interest – “and correspondingly larger increases in taxes on middle- and/or lower-income taxpayers.”

Deep cuts in spending would have a similar impact, the authors added, hurting mostly low- and middle-income households because they disproportionately benefit from government programs and benefits.

And to critics who have said the Center study did not assume enough economic growth from Mr. Romney’s proposed tax cuts of roughly $5 trillion over 10 years, the authors said their conclusions were essentially the same even when using growth estimates developed by Gregory Mankiw, a Romney adviser, a Harvard professor and former economic official in the George W. Bush administration.

The authors, in their follow-up paper, also answered commentators who said the Center should have assumed that Mr. Romney would end tax breaks for state and local bonds and for some insurance policies. By not including those among his protected tax breaks for investments and savings, these critics have said, he could raise additional revenues and avoid increasing taxes from low- and middle-income taxpayers.

The authors, in their response, illustrated that repeal of the two tax breaks for public bonds and for certain life insurance savings would not raise enough revenue. They added, “Our main result still holds.”