‘It’s a monster,” says the Ways and Means chairman, Charles Rangel, about the Alternative Minimum Tax. Senators Clinton and Schumer think so too. As they see it, if Congress does nothing, the muchmaligned AMT will affect 23 million upper-middle class taxpayers in 2007, up from about 4.2 million in 2006, and could hit over 36 million in 2010. Whenever New York Democrats cheer a “tax cut for the rich,” this man-bites dog story requires an explanation.

The AMT was created in 1969 to prevent 155 wealthy taxpayers from using deductions and credits to avoid paying any federal income taxes. But because it has been expanded and because it was not indexed for inflation, the AMT affects a growing share of the population.

Indeed, it delivered a significant tax increase to an estimated 3% of households this year. Unless the law is changed, it is projected to strike nearly 20% of taxpayers when they file returns next spring.

A Democratic Congress enacted the AMT to prevent people from taking too many deductions. Taxpayers subject to the AMT must calculate their tax liability twice: once under regular income tax rules and again under AMT rules. If liability under the AMT proves higher, taxpayers pay the difference as an add-on to the regular tax. The difference paid is their AMT.

Because it disallows certain tax breaks — especially the state and local tax deductions and the personal exemption — the AMT hits some taxpayers harder than others. Married couples with children and taxpayers in high-tax states are disproportionately hit by the tax.

And that’s why Democrats are so upset. About half of the people paying the tax in recent years live in one of four states — California, Massachusetts, New Jersey, and New York — accounting for almost a quarter of the nation’s population. This liberal tax hits liberal states really hard.

To add insult to injury, taxpayers also have to deal with the real bracket phenomenon. This phenomenon is even more pernicious in high cost places of living like New York City where some employers offer generous salaries to offset sky-high rents and other city-related costs. Since the inflation index used to set the brackets does not appropriately take these localized costs into account, the more people earn to make up for the high cost of living, the more taxes they pay, and the more likely they are to be hit by the AMT.

New York taxpayers are particularly burdened. According to Citizens for Tax Justice, New York was among the 10 states hit hardest by the AMT in 2006 with 17% of its taxpayers paying the addon federal tax.

According to recent data collected by Senator Schumer, in 2004 the AMT tax liability in the state was, on average, $4,900. Soon, it is likely to get worse. In 2007, New York will have the third highest state and local tax burden in the nation. In addition, the latest census data and survey of property tax collections show New York counties extracting some of the highest property tax payments in the country.

What really hits home with Senator Schumer is a study prepared by Martin Cantor of Dowling College that shows Long Island’s taxpayers — the senator’s home county — will be hit harder by the AMT than any other region in the nation, with the exception of Stamford-Norwalk Connecticut.

By 2010, the study finds, almost every Long Islander with an annual income above $60,000 will find themselves in the AMT’s crosshairs.

But Long Islanders are not the only taxpayers in the state affected by the AMT. According to the Tax Foundation, three of the top five most highly taxed Congressional districts lie in the New York City area. Mr. Cantor’s study also shows that the taxpayers in the New York City metropolitan area are the hardest hit in New York State in terms of taxes paid.

Of the nearly 2.2 million tax filers in New York City who have a tax liability, 127,200 will pay the AMT. This is 55.1% of the total AMT paid by New York State taxpayers.

More than 80% of households with incomes between $100,000 and $200,000 and almost half of those with incomes between $75,000 and $100,000 will pay the AMT by 2010, compared to 4.8% and 0.7%, respectively, in 2006.

At higher income levels, the share of taxpayers of the AMT falls, because the top AMT rate is lower than the top regular tax rate. So, after years of temporary “patches,” Democrats say they now want a permanent change.

In their fiscal 2008 budget blueprint, Democrats provided relief from the Alternative Minimum Tax. No couple with an annual income below $250,000 would pay the tax again, and that’s where it gets interesting. Taxpayers making between $60,000 and $250,000 can hardly be called middle class. According to the latest census income data income, if your household made more than $92,000, you are in the top 20%. Making more than $167,000? You are in the top 5%. Most people in both groups are aggregated in the major metro areas of the northeast and west, like New York City, Westchester, and Long Island.

Even by New York standards, the taxpayers that the Democrats would like to shelter can hardly be called needy. Does Mr. Rangel, who opposed the Bush tax cuts of 2003 because he thought they were too generous to the wealthiest Americans, only want to cut taxes for the “rich” — whom he calls middle class — if they live in his district?

In 1999, with no help from Rep. Charles Rangel, Congress passed a bill to repeal the AMT. President Clinton promptly vetoed it, allowing the monster to roam free. But, perhaps, almost 10 years of tax terror in the liberal countryside will convince Senator Clinton to slay the AMT monster.

Ms. de Rugy is a senior research fellow at the Mercatus Center at George Mason University.