A Joe Arpaio-style Republican would do even worse than usual for his party at the northern end of the district, though.
I'd surmise that the GOP has a much better shot of holding the seat by playing to the middle of the district, not the two pieces of the district that Patrick won. Brookline and Newton are not where this election will be won and lost. It'll be won and lost in places like Taunton.
Would we agree that you need a decent Republican environment for a Republican to win, with some comfortable upper middle class people turning their backs on the Dems? If so, that means lots of wealthy moderates in Needham, Newton, and Brookline who otherwise vote Democratic, voting for a Republican they consider safe. The Republican doesn't have to win that part of the district or come close, but he needs a) to get some votes there and, more importantly, b) not anger people so much that they come out in force against him.
You need a Mitt Romney or a Scott Brown, not someone who unabashedly plays wedge issues.