From a libertarian standpoint, the justification for repudiating the debt is fairly obvious -- see Murray N. Rothbard's essay on the subject or listen to last night's episode of Free Talk Live (MP3), for example.Knapp points out that politicians can easily ignore grass roots movements, as they have done with the bailout larceny. But serious talk of debt repudiation would spread fear among the elite: the "investors" in government debt won't like it, and that could spell trouble for politicians who believe the serfs can always be forced to pick up the tab to pay back the investors.
In terms of visceral, gut-wrenching populist appeal, debt repudiation has a lot of potential, too, and it's nice to see that appeal exploited. For a good example, see the YouTube campaign of kids (correctly) asserting "it's not MY debt."
Conveying the basic injustice of expecting someone else (especially a bunch of kids) to pick up the tab for spending that they had no part in is easy and effective, and the knotheads in Washington just keep making it easier with their bailouts, projections of future $1 trillion deficits, etc.
Take away that perception -- stir, in the buyer's mind, the possibility that those bonds may at some point in the near future become mere paper of no value whatsoever -- and that buyer is going to either stop buying or at the very least demand a much larger return for the risk.
And that is how we get at the politicians. They can always vote themselves the power to borrow money and try to stick the rest of us with the debt, but that power is meaningless if nobody's willing to lend it to them.