I can't believe I haven't figured this out yet. The closeness of the race in Massachusetts is not just a function of the dreadful Martha Coakley and right-wing hostility to health insurance reform. It's also about the Kennedys.
This Senate seat was held warm for Ted decades ago, when he was parachuted in and stayed there for ever. Part of the revolt is based on the fact that Coakley seems to be the ultimate Kennedy clan crony, and was also plopped in by a tiny number of primary voters, and seems to imbue the arrogance of the Democratic party elite. Most voters know that she could lord it over them for decades. But they'll almost certainly be rid of Brown in a few years.
Brown has also played class politics more effectively. Obama's swoop in to save Coakley also makes him look like an upper-class elitist rather than a mobilizer for change for the poor. The optics, as they say, are awful.
So you have resistance to machine politics in a seat controlled by elites for ever; you have an atmosphere of unrest and discontent after two years of recession; you have the Republican base whipped up into an FNC-induced frenzy against the end of America as they know it; and you have the Herald readers sick to death of Kennedy power. For good measure, you have the ugly spectacle of closed door final meetings in Washington over health reform.
I don't see how even Obama can turn back this perfect storm.