I've got some news for you: It doesn't matter in the slightest if Barack Obama -- or any other Democratic politician -- "advances the Reagan myth".
You know why? Because the Reagan myth isn't a hypothetical, some figment that American populace might someday accept or reject. It's already firmly embedded in the American culture, and it's not going to go away, no matter how many facts we throw at it.
The reason for this is very simple. Ronald Reagan won huge electoral victories (44 states against Carter, and 49 states against Mondale), and the American public needs a palatable explanation for this fact.
We may know here that the reality of Reagan was very different from the myth of Reagan -- that the real Reagan campaigned on fear and racism -- but the American populace is not going to admit this. Racism is taboo in modern society, and no one wants to think of themselves as motivated by fear. We do not want to see ourselves that way. We do not want to think of ourselves as cowards and villains. So we have collectively bought into the myth of Reagan as an icon of renewal and American spirit.
It really doesn't matter whether Democrats confirm that myth. We are not going to change it, not until we have a better story to replace it. (Frankly, until we do have such a story, challenging the Reagan myth might be counterproductive. It alienates us.)
What does matter is how we make use of the Reagan myth. If it's part of the culture now, then it's fair game. In this sense, I think Obama's rhetoric -- which is addressed at the mythical Reagan, not the historical one -- is good politics.
He's co-opting one of the right's most powerful stories, and re-purposing it for progressive use.