Impeachable Offense?

Al Franken has been making a good point on his show the last few weeks. From the Constitution of the United States:

AMENDMENT XIV
Passed by Congress June 13, 1866. Ratified July 9, 1868.

Section 4.
The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned.

In other words, it is unconstitutional to question the validity of a debt that the United States owes to another party. Now follow me here... US' debts are represented by treasury bonds. Other countries buy them, for example, and we can buy them. Guess what else they're used for - the money in the social security trust fund. Now. George W. Bush has been marching around the country the last few weeks railing against social security, calling it a bankrupt system full of worthless IOUs. If he thinks that treasury bonds are worthless IOUs, isn't he calling into question the validity of the public debt? Isn't he violating the constitution? Didn't he swear to uphold the constitution? I remember seeing his hand on a bible at some point...