Republican senate hopeful Scott Brown's Sunday rally featured Curt Schilling, Doug Flutie, and Cheers star John Ratzenberger (Cliff). Coakley's featured the entire Democratic establishment and the President (whose remarks seemed pretty tone-deaf). Whom would you rather show up for you? The key stat to remember is that Hillary beat Obama 56-41% two years ago in the Massachusetts primary (h/t Patterico). Moreover--and please correct me you descendants of the original Tea Partiers--Massachusetts politics remains white ethnic politics; they elect black politicians (Brooke, Patrick), but the racial edge remains.
Just consider the Boston Celtics, long regarded as a white team (just as the Cubs are in Chicago; Obama of course is a White Sox fan, as he said at the Coakley rally). I recall Clarence Thomas saying to a group many years ago that he experienced more racial animosity in Massachusetts than he had in Georgia.
To recognize such realities is of course not to endorse them. In this context, Obama's appearance may aid Coakley, but it could help sink her.
UPDATE: See Pete's comment below.
Ken, it is a little less white ethnic politics than it used to be and that could make all te difference. PPP has their voter screen (in their last poll, the new one is being posted tonight) at 14% African American and Latino. It has Brown winning among whites by about 12% but losing by huge margins among Latinos and African Americans. All this shakes out to a statistical tie ( a 1% Brown lead). If this were the Massachusetts of 1980, it would be tough to see how Coakley could win, but as it stands, the Democrats only have to slightly cut Brown's margins among white suburban persuadables to win.