Mitt Romney's campaign did exactly that with a fact-checked claim originally from Eric Cantor (R-Va.).
When PolitiFact went to fact check Romney, it ended up fact checking its own work and finding it wanting.
So we find that “PolitiFact” changed the basis for their assessment once Republicans started using their own assessment to beat up their boyfriend Barack. So will they start giving out ratings of their own posts?Visit PJ Tatler for all the gory details.
The evidence of ideological bias would improve if the comparison wasn't between two Republicans, but Poole makes a good point about PolitiFact fact checking itself. PolitiFact is building a record of wild inconsistency, and it appears to break predominantly against conservatives.
Changed Tatler links to put focus on Tatler story rather than my added commentary.