The easiest way of defaulting on the part of the social security promise, that is associated with the trust fund, is to let it grow indefinitely. For that, there has to be made the steady beat, that it is underfunded. The claim, that soc sec has to be privatised is useful for the Republicans, as long as it is not really done.

Publicly the deficit of the unified budget is usually cited, covering up, that the general tax income just is enough for 2/3 or so of the general budget. A couple of years ago the situation may have been different. The idea of the Republicans may have been to plunder the gov't to the benefit of the financial industry. But now it is pretty clear, that a crisis of the general budget is unavoidable. To counter that, there would have to be massive tax increases in the very near future, if the promises of the soc sec trust fund would have to be paid out. What is left to plunder, is now plundered by other schemes.
There maybe left over Republican talking points from the past, or they simply don't have a clue. Privatisation of soc sec now would blow of the whole game.

And which position is weaker is not so clear. Given current circumstances, I see the protection of private investors - as well small investors, bond holders of banks probably are very often private retirement fonds, to the disadvantage of the public, in which the subgroup of people with claims on soc sec are especially vulnerable to a fiscal crisis. Being part of a voting bloc seems not to be such a strong position, when Dems and Reps unite against the interests of such a bloc.

By the way, I'm not aware of any public European retirement system, that takes part of the pay roll tax as saving. In Germany it is even vice versa, that the general budget is used to bolster retirement.

Der Amerikaner ist die Orchidee unter den Menschen
Volker Pispers