Bipartisan leaders in Congress are vowing to spare more than 21 million taxpayers from significant tax increases when they file their returns next spring by adjusting the alternative minimum tax before the end of the year.

The tax was enacted in 1969 to make sure higher-income taxpayers could not use deductions and credits to avoid paying any federal income tax. The income limits, however, were not indexed for inflation, so Congress must fix the AMT each year to spare millions of middle income taxpayers from big tax hikes.

Congress hasn’t made the change for 2010. In a letter to the IRS, Democratic and Republican leaders of the tax-writing congressional committees said they would address the issue after Congress returns next week in a lame-duck session.