WASHINGTON — An estimated 20.5 million Americans could be subject to the 28 percent alternative minimum tax in 2006 because Congress won't be able to enact a one-year fix before adjourning for the year.

The tax, which disallows some of the deductions claimed by affluent filers who itemize on their returns, is expected to be paid by an estimated 3.8 million filers for the 2005 tax year, according to the Treasury Department.

Next year, it could hit a family of four earning as little as $58,500, according to the nonpartisan Congressional Research Service. Additionally, the AMT could undo much of the effect of the Bush administration's tax cuts enacted since 2001. A family of four earning $80,000 could end up paying $1,585 more in taxes, according to CRS.

Senate Majority Leader Bill Frist, R-Tenn., gave reporters the official word of the postponement Tuesday, although it already was widely expected that the Senate and House would not be able to reconcile two significantly different approaches to expiring tax breaks.

Even though Congress will have missed a deadline, it won't affect taxpayers when they file their 2005 returns in early 2006.

And there's plenty of time to act before 2006 tax returns are filed in early 2007.

The AMT works under a formula that disallows federal tax deductions that are considered to be excessive for tax filers with incomes above certain thresholds.

For affluent Americans who still consider themselves middle class — earning $90,000 to $120,000 annually in high-cost-of-living states such as New Jersey and New York where the AMT kicks in most frequently — it already is a nuisance many consider unfair.

Grace Roossien, a financial adviser in the western New York community of Vestal, said the AMT disallows as a charitable deduction part of the 10 percent of income she tithes to her church, Valley Christian Reformed Church in Chenango Ridge.

"It's a sizeable chunk of money," Roossien said.

She said that people in her community who moved into upscale new homes found the high property taxes they pay are not fully deductible on their federal income taxes because of the AMT.

According to IRS data for 2003, the most recent year available with a state-by-state breakdown of who pays the AMT, the tax hit more than 4 percent of filers in New Jersey and New York, and it affected more than 3 percent in Connecticut, the District of Columbia and California.

The number of filers affected has grown almost 60 percent over the past two years, from fewer than 2.4 million tax filers in 2003 to an expected 3.8 million this year, according to the IRS.

House Republicans dubbed the Alternative Minimum Tax the "stealth tax" in a bill passed last week that would extend the current income threshold for one year with a small inflation adjustment.

However, the House bill was not part of a larger tax package that is supposed to be reconciled with the Senate. The tax package passed by the Senate had a more generous one-year extension that exempted 600,000 more tax filers than the House bill.

For example, the Senate bill exempts married couples earning less than $65,550, up from the current exemption of $58,000. The House legislation would set the threshold at $59,600.

The President's Advisory Panel on Tax Reform recently proposed repealing the AMT as part of a tax-simplification plan. To offset the lost federal revenue, the panel proposed eliminating the deduction for state and local taxes as well as scaling back the deduction for home mortgage interest with a smaller tax credit.

Repealing the AMT is an attractive proposal to David Kotok, chief investment officer of Cumberland Advisors in Vineland, N.J.

"The AMT costs me personally an extra $2,000 to $3,000 a year in accounting fees," Kotok said in a telephone interview. "I have a complex return." He said a repeal of the federal tax deduction for state and local taxes would be a worthwhile tradeoff that would focus citizen attention on high state and local taxes.

Under the current system, Kotok said, some of his clients have left New Jersey to escape the federal AMT and moved to lower-tax states such as Pennsylvania, Florida, Nevada and Wyoming.