Joe Biden is telling seniors that Romney will increase taxes on Social Security benefits, in order to help finance the high-end income tax cuts that Romney claims he can enact on a revenue-neutral and distributionally neutral basis.
Biden is basing this on an Obama campaign memo that says it it would be “impossible for Romney to pay for his plan without also cutting deductions [sic] that the middle class relies on,” including the exclusion for Social Security.
For good measure, the Obama campaign memo adds: “It’s no surprise coming from Mitt Romney, who has written off half of America — including seniors — for receiving a government benefit, even if they spent a lifetime earning it” (a reference, of course, to Romney's infamous "47%" slur).
Would Romney actually do this? Who knows? It's unmistakable that he would have to take a meat axe to a whole bunch of other popular items that he refuses to identify by name, such as the home mortgage interest deduction and the exclusion for employer-provided health insurance, in order to get anywhere close to the target. And if that is the nature of the exercise - as it would have to be, purely as a matter of arithmetic - then why wouldn't Social Security benefits be on the chopping block? It wouldn't be a first, after all. The benefits became partly taxable in 1983, and additionally so in 1993, albeit in each case only for people with sufficient adjusted gross income (which sounds like the dreaded "redistribution"). And making them fully taxable would indeed increase the percentage of Americans who are paying some federal income tax, which many Republicans have said is an important policy goal (a position that Romney's "47%" riff implies that he shares).
Even if Romney and his advisors secretly plan not to increase income taxation of Social Security benefits, they have no right to complain when others speculate to the contrary. You can't advertise the good news (the rate cuts), refuse to say anything whatsoever about the bad news (the pay-fors), do so on a basis that is not credible (that they don't want to dictate the entire legislative package in advance - which is wholly consistent with listing the areas you are willing to cut) - and then feel ill-used when the other side fills in the blanks.