Obama has essentially framed the Democratic Primary debate in such a way as to ensure the only obvious contrasts are the ones that favor him. Regarding the contrasts that would favor Hillary, Obama has either neutralized them by saying: "me too!," -- or completely undermined them (for example, Hillary can't say she's more experience than Obama, because he will argue her experience involves voting for war in Iraq, supporting NAFTA, etc.).
In essence, the only difference Democratic primary voters are seeing are the ones that favor Obama. Because the two essentially agree on liberal ideology, the only difference between the two candidates is that one of them is way more charismatic and exciting.
Of course, this will not work against McCain. Obama will be forced to defend his Leftist ideology, such as wanting to socialize healthcare, precipitously withdrawing from Iraq, etc.
And based on the early skirmishes the two candidates have had, it is clear that McCain gets under Obama's skin in a way that Hillary never did, or perhaps, never could. Remember, Obama has had few tough races. For his U.S. Senate, he was challenged by Alan Keyes. His success belies his inexperience; He's never had a tough General Election opponent. Until now.
Against Hillary, he has appeared staid. But unlike his campaign against Hillary, when refuting McCain's attacks, Obama seems meaner and even disputatious. I'm not entirely sure how this will play in contrast to his image as a "new kind of candidate."
But the real question will be whether or not Obama can stand the Republican onslaught to attempt to portray him as the liberal that he really is. Michael Dukakis, you may recall, was winning in the polls until he was revealed to be a "card carrying member of the ACLU" who wouldn't support the death penalty even if his wife was raped and murdered, and who had released Willie Horton on a prison furlow.
It remains to be seen whether or not the American public will be willing to overlook Obama's liberal positions. Right now, though, it's clear most people don't even know about them. And Hillary sure isn't going to make an issue out of them. Obama looks great today, but is this his abogee?