Stan Greenberg, Al Quinlan and James Carville lay out the good news for Democrats in their latest survey before delivering a warning to the party and its presidential candidates:

For all that, Democrats have yet not found their voice as agents of change, except perhaps on Iraq, and risk falling short of their greatest aspirations. [snip]

Democratic candidates for president and Congress are polling at 51 to 53 percent in the
various races, but if 2008 is to bring a tidal wave, Democrats and progressives must become more fully the voice of what is wrong with these times. It is not enough to be anti-Iraq and
anti-Bush. The conservative attack machine will soon launch nuclear war against the Democrats' nominee when he or she emerges, and a lot of the discontent in the country could
fragment and push voters to third-parties and some even back to the Republicans, particularly if
progressives fail to tackle key grievances, like immigration and taxes. Seats and states that
Democrats now covet could be lost without a clearer articulation of the Democrats' critique of
the times and their willingness to be agents of change.[all emphasis in the original]

The "change agent" argument would seem to fit Obama's greatest potential strength, but so far it's a strength that hasn't been realized, at least in most of the poll data we've seen.