He was elected in 2008 in an overwhelmingly Democratic and African-American district in New Orleans. And the only reason he was able to do so is that he was running against Rep. Bill Jefferson (D-LA), who at that point was under indictment but not yet convicted of various public corruption charges, the most well-known of which was tied to that notorious $90k found in his freezer. To understand Jefferson's political position at that point, remember that the crux of the accusations against him first surfaced in 2005. So despite his popularity and strong political base at that outset, his constituents had had three years to absorb the seemingly overwhelmingly evidence of his corrupt acts.
Cao's ability to hold that seat against a non-indicted Democrat seems highly questionable. It went 75% for Obama. And the White House has been in close touch with him through the day. An aide tells Roll Call, "Rahm is going all in to get him."
Those would be some interesting conversations to listen to.