WASHINGTON — The Bush administration and Republicans on Capitol Hill are warning that the Democratic leadership’s failure to confront the alternative minimum tax could affect nearly 50 million tax filers, causing a potential “filing fiasco” and holding up as much as $75 billion in refunds next year.

The chairman of the House Ways and Means Committee, Rep. Charles Rangel of Harlem, is set to introduce a broad tax proposal as soon as tomorrow, including a measure to “patch” the minimum tax for one year.

Democrats must move swiftly to pass the measure, or Mr. Rangel may have waited too long, the administration and GOP lawmakers say. “To avoid confusion and delays for taxpayers, it is critical that an AMT patch be enacted by early November,” the treasury secretary, Henry Paulson, wrote in a letter to senior lawmakers yesterday. He said 25 million taxpayers could face a “substantial unexpected tax increase” if Congress doesn’t act, a figure that is 1 million to 2 million more than previous estimates.

“Enactment of a patch beyond early November could also significantly delay processing of these taxpayers’ returns and payment of any refund,” Mr. Paulson added. He also said that if Congress does not address a separate rule that applies to many popular tax credits, an additional 25 million taxpayers could face delays.

The reason is fairly simple: The Internal Revenue Service must finalize the various tax forms by early- to mid-November, and any changes to the 2007 code made after that would create a bureaucratic nightmare.

Leading Republicans released Mr. Paulson’s letter to reporters yesterday, seizing on it to accuse Democrats of dragging their heels. “The Democratic leadership needs to step up to the plate — and do so now. They have the responsibility,” the ranking Republican on the Senate Finance Committee, Senator Grassley of Iowa, said. Further delays, he added, would cause a “filing fiasco” on April 15.

Lawmakers in both parties support the patch as well as an effort to scrap the AMT permanently, but Republicans do not want to pay for it by raising taxes on other Americans, as current “pay-as-you-go” rules enacted by Democrats require. Estimates place the cost of a one-year fix between $65 billion and $80 billion, while a permanent elimination of the tax would cost more than $840 billion over a decade. The problem has lingered because although the AMT was first created to ensure that only the 155 wealthiest Americans paid their fair share in taxes, it was never adjusted for inflation and could hit the middle class if Congress doesn’t step in.

Democrats dismissed the warnings from Republicans as little more than political theatrics. “Republicans are overreacting in a transparent attempt to score some political points,” a spokeswoman for the House majority leader, Rep. Steny Hoyer of Maryland, said. “Democrats are the party of the middle class and we are not going to let hardworking Americans face increased taxes through the AMT.”

Mr. Rangel has said he has received assurances from Mr. Hoyer that the one-year AMT patch would be brought to the House floor quickly once he introduces it, but no specific timetable has been announced. He will simultaneously unveil his much broader tax overhaul — which he has dubbed the “mother of all tax reform” — but he has acknowledged it is unlikely to be voted on this year.

Mr. Grassley and the ranking Republican on the Ways and Means Committee, Rep. James McCrery of Louisiana, declared their blanket opposition to Mr. Rangel’s forthcoming proposal, which will include significant tax increases to pay for a permanent repeal of the AMT, as well as a one-year patch. “If it’s paid for, so to speak, it’s a tax increase, and I would oppose that,” Mr. McCrery said.

Mr. Rangel’s bill is expected to include a surtax on the highest-earning Americans, most likely those making more than $400,000 a year. The chairman also confirmed to Bloomberg News in a television interview yesterday that a tax hike on the “carried interest” profits earned by managers of hedge funds and private equity firms would be included in both the broader bill and the more limited one-year AMT fix.

The Wall Street Journal reported yesterday that the bill would also seek to reduce corporate tax rates below their current 35% and would pay for the decrease by eliminating a tax deduction for manufacturers.