Another thing being ignored: The effect of Ted Kennedy's primary challenge, which probably explains much of Carter's troubles in the summertime. Kennedy did not give up until the day of his convention speech.
Take away the Ted Kennedy factor, and Reagan probably would have trailed the entire year until the debate.
So, you're talking about a 1980 what-if. LoL. What TL did you read before posting this? Some that had Carter unopposed in the dem primary, or what?
Oh, and 1980 =/= 2012. We have internet, mobile phones... they had libertarians and democrats for Reagan. Carter was an unispiring candidate, Obama isn't. Reagan was a man the conservative base loved, Romney isn't. That year they had Anderson, now we have Goode and Johson. And something you are forgetting to mention. Obama is still leading in key swing states, and his lead is not exactly dropping. At this time, in 1980, the race was leaning Reagan.