Obama vs Carson.

So way back in 2008 when Obama was running, I heard the words "if you don't vote for Obama you're racist" said many times (even my parents said it). I also heard the words "I'm voting for Obama because he's black and we need a black president" MANY times. Now here's my question, there is a black man running for office again (Dr. Ben Carson) and all I hear is people calling him an Uncle Tom. What happened to voting for someone because they are black like in 2008? A change in party suddenly stops the race card? Can Carson argue that he didn't win the presidency because he's black and America is racist? Curious to know why one party can do it but another cant?