It’s a lesson that Republican tax-cutting zealots are now learning, painfully, as their crusade to minimize taxes for the ultrawealthy teeters on the brink of failure. As we’ll see, this lesson applies to both parties: What’s happening to the tax-cut zealots today could happen down the road to the Democratic zealots who insist that Social Security doesn’t need to be fixed.

Taxes first. The reason we’ve got a fiscal cliff is that back in the early aughts, when people like Alan Greenspan were worried that federal budget surpluses would become excessive (you can snicker now), President George W. Bush came up with a seemingly brilliant strategy to force through enormous tax cuts. Rather than compromise with Senate Democrats to get substantial permanent cuts, the Bushies used a legislative process called “reconciliation” to push through mega-cuts with the narrow Senate majority they held, avoiding the need for 60 votes to overturn a filibuster (had the forces of fiscal sanity decided to mount one).

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Because of the way reconciliation works, the 2001 Bush cuts had to expire in 10 years (eight years for the huge 2003 cut in rates on investment income). But hey. At some point, the zealots figured, they’d get the votes to make the cuts permanent. And even if they didn’t, by the end of 2010 no one would dare let the cuts lapse. We would be in an economic boom, thanks to lower taxes. Life would be grand, especially for the mega-upper-classes, with no estate tax and the lowest taxes in the modern era on investment income.

Well, that didn’t quite work out, did it? The eight Bush years were an economic and fiscal sinkhole, and the first two Barack Obama years weren’t exactly robust. When the Bush cuts expired at the end of 2010, Republicans bought a two-year extension by agreeing to support extended unemployment benefits, and payroll tax cuts. But now the day of reckoning has come.

The biggest cuts for the biggest incomes seem certain to disappear. What’s more, rates for those folks are likely to be higher than they would have been if deals had been made in 2001 and 2003.

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We’d have no cliff drama at all had Bush done a truly bipartisan tax cut with the likes of Sen. Kent Conrad (D-N.D.), a fiscal conservative who’s one of my favorite politicians. Make that former politician — he’s leaving the Senate soon.

Says Conrad, who opposed the Bush cuts’ excesses: “They bet the farm, and we all lost. It’s coming back on them, and it’s coming back on the country.” How does he feel about having been proved right? “I take no joy in it at all,” he said. See why I like him?

I now fear that Democrats, currently feeling as empowered as the Bushies felt a dozen years ago, will make the same mistake with Social Security that the Bushies made with taxes.

Just as Bushies used ridiculous numbers to justify tax cuts, the official Democratic position uses ridiculous math to claim that Social Security doesn’t need fixing. The logic, such as it is: Social Security isn’t a problem because it shows a federal budget surplus. (I don’t know if there’s a different, unofficial position. I sure hope so.)

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But our country’s real fiscal problem isn’t the federal budget deficit. It’s the huge increase in debt our country is incurring to pay our bills. In the fiscal year ended Sept. 30, the Treasury had to borrow $160 billion (including $112 billion for the payroll tax holiday) to cover Social Security’s bills, even though the program showed a $65 billion budget surplus. (I explain the math below.)

Right now, Democrats are riding high. But if they don’t work out relatively modest, bipartisan cuts of future Social Security benefits now, in a decade or two the political climate may have changed, Social Security’s problems will be worse, and cuts will be bigger and nastier than if they’re made now.

Repeat after me: What goes around, comes around. Always has, and always will.

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This story is from the January 14, 2013 issue of Fortune.