The Social Security tax is a tax on labor income. Because it is “capped” (it applies to only the first $106,800 of wages for 2011), it is not a tax cut on the margin for the people who make the decisions that determine economic growth. And, the tax cut passed by the Senate was to last for only two months.
So, what we have here is a tax cut that does (at best) nothing for economic growth, does (at best) nothing for employment, but adds more than $100 billion per year to the deficit and debt. Why do most of the Democrats and many of the Republicans in Congress support it? Here is one explanation.
America has a two-party system. We have a Stupid Party (the Republicans) and an Evil Party (the Democrats). Every so often Congress does something that is both stupid and evil, and we call this “bipartisanship”.