Alternative Minimum Tax

The AMT was introduced by the Tax Reform Act of 1969 and became operative in 1970. It was intended to target only 155 high-income households that had been eligible for so many tax benefits that they owed little or no income tax under the tax code of the time.

The AMT disallows many deductions and exemptions allowable in computing "regular" tax liability. (Regular tax liability is defined in 26 U.S.C. § 55(c)(1), with reference to 26 U.S.C. § 26(b), and does not include AMT and various other categories of taxes imposed under Chapter 1 of Subtitle A of the Internal Revenue Code.) The AMT sets a minimum tax rate of either 26% or 28% (depending on the amount of the taxpayer's "alternative minimum taxable income," as adjusted) on some taxpayers so that they cannot use certain types of deductions to lower their tax. By contrast, the rate for a corporation is 20%. Affected taxpayers are those who have what are known as "tax preference items". These include long-term capital gains, accelerated depreciation, certain medical expenses, percentage depletion, certain tax-exempt income, certain credits, personal exemptions and the standard deduction.

Basically it taxes people who make over a $100,000 on a flat tax basis, to prevent too many deductions. But it was not tied to inflation, and as wages increased (slowly, over time) the tax affected to many people. McCain plans to remove the tax as a result, while Obama wants to index it to inflation. Obama wins because indexing this tax to inflation continues to ensure that the wealthy don't get out of paying any tax by abusing tax loopholes.

Some Info From Wikipedia

2008 Presidential Election
Comparing the candidates on the issues.