“The absence of vocal opposition has more to do with their fear of challenging the legitimacy of the “first Black President” than anything else. They are terrified of the media crucifixion they would endure if they challenged and failed. Their bravery lies in the same direction as their own personal interests, Not with objective truth. THAT might require some sort of sacrifice, which they are loath to provide.”
And besides, he’s already a term-limited lame duck.
But no one can attach the "You are just doing this because you are a RACIST!" accusation to any of these other issues. Citizenship and Legitimacy issues tend to direct attention to a sore spot in our history; The Dred Scott decision. No one wants to get near it, and they would rather ignore the truth than suffer accusations of racism.
They know that the Media would label them as motivated by racist sentiment, and that most of the ethnic population as well as white Liberals would support this accusation. They see it as a social meltdown, and so they would rather not examine too closely, or pull that thread. It would be like pulling the lanyard on a societal cannon.
They don't touch it because they fear it. He is shielded from attacks on this issue because of his ethnicity, and not because there is no merit to the accusations.