I suspect that the meaning of "it" here is the fact of lame duckitude: the lame duck is the last to figure out that he is a lame duck. Of course, it all depends on what the meaning of "it" is (to quote a great philosopher).
Heck, I didn't think it was all that philosophical or hard to figure out. Shelby Thames may be the last person to realize that he's a lame duck. Everyone else knows it and, in knowing it, assures it. Thanks for responding to query on my behalf, USMS.