For conservatives it was the idea that the individual mandate was unconstitutional. That idea has been on the table because it satisfied the deep desire among conservatives to continue the battle against the Affordable Care Act after their legislative defeat (and before their hoped-for [by them] legislative/executive victory to come in 2012).
For liberals it appears to be the idea that legislation setting a debt ceiling is unnecessary because of Section 4 of the Fourteenth Amendment.* Is that an off-the-wall argument? One test might be this: Can anyone locate any writing, preferably legal scholarship but even off-hand musings will do, making that argument prior to, say, November 5, 2010? [I derive the test from thinking about how the so-called Article II argument in Bush v. Gore developed.]
To be clear, this isn't a criticism of the Section 4 argument. Politics gives people incentives to make innovative constitutional arguments. What's interesting for me, at the moment, is whether those incentives are strong enough to move the argument off the wall (if that's where it was last year) on to the table. Those incentives were strong enough to put the individual-mandate argument into play (to shift the metaphor). Will the incentives on the liberal side be strong enough to put the Section 4 argument into play?
(Until an argument is in play -- or on the table -- it's not really worth the time it would take an outsider [in the present context, someone like me] to evaluate the argument's merits. That's a point about how one allocates one's time, not a point about off-the-wall arguments as such. Qua arguments, those arguments should be evaluated on their merits. But, really, how much time and energy do Randy Barnett and Jack Balkin devote to evaluating the merits of tax protestors' arguments against the constitutionality of the federal income tax, or the jurisdiction of federal courts to adjudicate anything other than admiralty cases, if, as I vaguely recall, that's in the tax protestors' arsenal of arguments.)
* That seems to me the right way to put the argument, rather than saying that a statute setting a debt ceiling is unconstitutional because of Section 4. But, that gets close to discussing the merits of the Section 4 argument, which is something I want to avoid.