Here's the problem with extending the payroll tax cuts: They're unlikely to ever be restored. And if they're not restored, you've done what Republicans have been trying to do for decades: turned Social Security into a welfare program that no longer pays for itself, but comes out of the general fund and for the first time, adds to the deficit.

So when you hear people saying it's a good idea for the economy, they're right -- for the short term. It's additional stimulus at a time when it's badly needed. But for the long run, it will politically undermine the long-term future of Social Security -- which the administration probably considers a feature, and not a bug:

The President asked for a $175 billion one-year extension and expansion of the employee payroll tax holiday now in place, halving the tax rate to 3.1 percent in 2012. He also proposed halving employer payroll taxes to 3.1 percent for the first $5 million of payrolls in 2012. The president also wants a complete payroll tax holiday that would apply when companies grew their payrolls by up to $50 million in a year by hiring new workers or raising the salaries of existing workers.These cuts in the Federal Insurance Contributions Act tax (FICA) may be one of the best available stimulus options in the current political climate, and they will have a positive economic impact.

Ananalysis by The Center for Budget and Policy Priorities notes that the cuts already in place make a substantial difference in the spending power of middle class families, and that allowing them to expire at this time would be very negative for growth:

Failure by Congress to extend the temporary payroll tax cut enacted last December would reduce all paychecks starting on January 1, withdrawing needed support from the still-weak economy. The measure, part of the tax cut-unemployment insurance deal between President Obama and Republican leaders, reduces the employee share of the Social Security payroll tax,[1] boosting workers’ take-home pay by an estimated $120 billion in 2011. The tax cut is worth $934 to the average worker.

And Moody’s Analytics estimates that allowing the payroll tax cuts to expire would reduce GDP growth by one percentage point in 2012, translating into one million fewer jobs by the end of next year.

But Social Security advocates worry that these temporary payroll tax cuts will never be restored. “The problem is, it is very easy in our current political climate to cut revenue and very hard to increase it,” says Nancy Altman, co-director of the Strengthen Social Security coalition and author of The Battle for Social Security, an excellent history of the program and its politics.

“Look at the controversy over ending the Bush tax cuts, which would only affect a small portion of taxpayers,” Altman says. “In this case, if you propose restoring the payroll tax down the road, you’d have to double the rates on workers making minimum wage. This is being sold as temporary, but it’s not likely to work out that way.”