No.
To say Palin caused the loss of the senate is not constructive criticism, it’s a fabrication.
It’s an attempt to make the loss of the senate an issue against Palin, because the agenda to stop Palin is sorely in need of ammunition.
Those are not exclusive; the charge is a constructive criticism, and in this case, it is not accurate. So I guess you could argue that any charge that turns out not to be true ceases to be constructive criticism, but you really seem to be arguing that making the claim was wrong, not that the claim itself was wrong.
And I seriously doubt that Bachus is part of this nefarious plot to stop Sarah Palin. He seems to really like her, and has said nice things about her.