Attack them at their strong point
Some people may be wondering why Bush has chosen Social Security to be his #1 target at the beginning of his second term.
Well, first of all, you have to understand the conservative program to undermine the entire social welfare state. It started with the deliberate bankrupting of the federal treasury. It will be completed only when the crown jewels of the social welfare state are torn down. The "we can't afford this anymore so lets stop it" phase was inevitable. The question was where to begin?
They could have gone after it piecemeal, biting off pieces at a time around the edges, like a shark circling a crowd of swimmers. But that kind of piece by piece action takes a lot of time and a lot of work. It would also be counter-productive since there would inevitably come a time when the swimmers would pool their resources to defend themselves against the shark ("Smile you son-of-a-bitch!")
The other strategy, taken from Karl Rove's playbook, is to defeat an opponent by going after their strong point first. We saw this very well during the 2004 campaign, when the Republicans brought down John Kerry by attacking his previously unimpeachable war record. It may seem like a risky strategy, but it works a lot more often than most realize. It just takes a certain ballzyness to do it (and Rove's got just the balls for the job). And when it works it can shatter the opposition into a thousand scattered pieces that can then be easily mopped up.
When it comes to the social welfare state, there is no bigger target than Social Security. If the Republicans succeed in taking it down than no element of that system will be safe. Social Security is the Superman in the Democratic Justice League. Take him down and whose going to stop the villains? Aquaman?