An expansion in the reach of the alternative minimum tax is the element of the U.S. fiscal cliff with the largest immediate effect on taxpayers and the most bipartisan appetite for a solution, creating the possibility that lawmakers could use it to propel Congress toward a deal.

If Congress doesn't act to prevent the $92 billion tax increase, the number of households facing the alternative tax would increase to 32.9 million from 4.4 million, according to the Internal Revenue Service. That's an average unanticipated tax increase of about $2,800.

The effect from the AMT, as the parallel tax is known, would be immediate in early 2013 because Congress hasn't addressed the change for tax year 2012, and taxpayers start filing returns in January. A retroactive AMT change is much more cumbersome than retroactive changes in the 2013 income tax rates, which can be handled through paycheck-withholding adjustments, said Kenneth Kies, a Republican tax lobbyist in Washington.