WASHINGTON - Congress acted yesterday in its final hours before holiday adjournment to block growth of the alternative minimum tax. The step puts off an economic hardship affecting more than 20 million taxpayers and avoids what would have been a political black mark for both parties.

The House voted 352-64 for a one-year fix of the AMT, a four-decade tax that originally was meant to touch only superrich tax dodgers, but that now strikes millions of households with middle and upper-middle incomes.

Without that fix, an annual ritual of Congress, those subject to the tax would have risen from 4 million in 2006 to about 25 million in 2007, with the average levy of $2,000 a taxpayer.

"What we are hearing across the country today is a collective sigh of relief," said Rep. Eric Cantor (R., Va.).

The legislation now goes to President Bush, who has said he would sign it because - as requested by him and other Republicans - the measure does not include tax increases or other new sources of revenue to pay for the $50 billion cost of the AMT relief.

The last-minute nature of the vote on the AMT fix resulted from a fundamental difference between the House and Senate. House Democrats had insisted that the $50 billion in tax relief resulting from the one-year fix must be paid for by an equivalent amount of revenue elsewhere, mainly by closing a loophole on offshore tax havens.

Senate Republicans, however, blocked the Senate from taking up legislation that includes a tax increase, and Bush threatened to veto any bill that raised taxes.

On Tuesday night, the Senate for a second time rejected the House-backed approach of a paid-for AMT bill. The House Democratic leadership, which was committed to paying for the tax relief, had asked the Senate to make one last stab at the issue. The Senate vote was 48-46 for the House bill, 12 short of the 60 needed to approve it.

With that vote, the House had no choice but to take up the Senate bill, which shields about 21 million taxpayers without a means to cover the cost to the Treasury.

"Let me be clear: There is no disagreement between Republicans and Democrats over protecting the middle class from the AMT," House Majority Leader Steny Hoyer (D., Md.) said. "The question is, will we do so responsibly or charge tens of billions of dollars to our grandchildren?"

Cantor, the Virginia congressman who is the Republicans' chief deputy whip, said yesterday's vote was "a huge victory for us."

The GOP position, he said, "has been consistent down the line. We don't believe we ought to raise taxes to correct the mistake of AMT."