Washington ? During the hectic, closing weeks of this session of Congress, leaders of both parties say they are determined to pass legislation that would prevent 23 million middle-income Americans from being hit with a tax increase originally designed to target only the super-rich.

But chances are slim that Congress will find the $50 billion over 10 years needed to pay for that legislation, in large part because a proposed tax increase on Wall Street firms and their managers lacks enough support in the Senate. A sprawling, big-money lobbying campaign appears to have succeeded in preventing the proposed tax increase on hedge funds and private-equity firms.

Tuesday, President Bush repeated his demand that Congress quickly repair the alternative minimum tax (AMT) so the middle-class would be spared an average $2,000-per-family tax increase on 2007 income taxes, and refunds of up to $75 billion could be distributed without delay. The longer Congress dallies in approving the fix, Treasury Department officials warned, the later those refunds would be.

In response, congressional leaders declared that the AMT “patch” was must-pass legislation. “AMT relief is going to get done this year,” said Jim Manley, a spokesman for Senate Majority Leader Harry Reid, D-Nev. “It has to pass,” agreed Charles Rangel. D-N.Y., chairman of the tax-writing House Ways and Means Committee. “I don’t see how anybody, Republican or Democrat, can go back home and point the finger at each other.”

But an effort by Reid Tuesday to bring the issue to a vote in the Senate failed when Republicans rejected his offer to vote on any one of three specific AMT proposals. He will try to force a vote on the issue as early as Wednesday.

Both parties expressed doubt, however, that enough votes could be mustered to approve tax increases sizable enough to replace the revenue lost from the patch. Republican senators and a growing number of Senate Democrats have been echoing Bush’s preference for the AMT to be altered without offsetting tax increases.