Why are the Republicans out to "get" Social Security?

The answer is simple. They have been against it since Social Security was first passed, they have tried to kill it every time then have come to power since then, they have attempted to run against Social Security in order to gain power, and the Bush administration is simply one more turn of the crank. Or to quote "Ben in Madison" at Daily Kos:

"Every time they've gained control of the government in the last sixty years, they have without hesitation turned their guns on the New Deal. They always do it, and they always lose."

Ben in Madison gives the whole, sordid history. Go read it.

Here is the short version from a dKos comment by JK Minnesota:

In 1936, 99% of House Republicans and 63% of Senate Republicans against founding Social Security.

In 1939, 75% of Congressional Republicans voted against Social Security as we know it.

In 1950, 89% of Congressional Republicans voted against Social Security as we know it.

In 1956, 86% of Senate Republicans voted against Social Security as we know it.

In 2005, how many Republicans will vote to end Social Security as we know it?

The pattern is clear:Republicans consistently vote as if Retirement Security should be reserved for only the very wealthy in America.
Jeremy Kalin for MN House
by JK Minnesota on Mon Mar 7th, 2005 at 15:48:23 CST

I would add

In 1964 Goldwater ran against Social Security.

Since the Reagan administration, the CATO Instutute has been against Social Security.