The debt ceiling, which would otherwise curtail a budget that the Congress just adopted. They couldn’t get all the cuts in the budget the first time, so they’re attempting another bite of the apple.

At the same time, they’re looking to avoid sequestration because it would mean across the board cuts - an austerity move, precisely because it would hit everything, including things they hold dear, like government jobs programs (defense and construction to name but two). They’d rather focus all the cuts on to the safety net, even though SS is paid for via the payroll tax that was just restored to its prior level after a two-year temporary reduction.

Restoring the tax to the 6.2% level means that beneficiaries down the road will have the benefits waiting for them that they expected to be there. It’s a solvency thing - and keeping the rate lower would have reduced the solvency, and hastened the need to cut benefits or make other adjustments (all in the intention of starving the program out of existence eventually). The GOP saw fit to allow the payroll tax to expire because they thought they could gain politically by saying Obama raised taxes on most Americans - even though the GOP and Congress as a whole approved the measure’s temporary nature.