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.