Obama never used the word "bus." He said in the back PERIOD. as in the punctuation mark "."
He was alluding to the metaphor republicans have widely used of the state of the nation being a vehicle and the politicians in power being the drivers. The republicans started that, once again. He alluded to that saying the middle class is riding passenger seat, and the republicans can sit in the back (period). No implying the bus scenario. That was a "faux" analyst who went that way, and then a African-American Republican using it in his campaign.
It's a bad metaphor for either party to use as it can be seen as referring back to the days when the white driver controlled or drove the bus, the white passengers rode up toward the front and the blacks had to "ride in the back". Neither party should use it as it can be interpreted as racist or demeaning. Obama was clearly stating that he/democrats were in charge, the middle class could set up front and the Republicans had to sit in the back meaning they were subservient. I grew up during the days of segregation and knew his comments would be spun the way they did. Obama being black or 1/2 black made it even easier to spin as Republican Steele could act insulted as he is black and doesn't want to ride in the back.