Malden, Mass.

With the race to fill John Kerry's vacated Senate seat entering its final days—the special election is Tuesday—Republicans and Democrats have memories of Scott Brown's upset victory in 2010 firmly in mind. But this election differs markedly from that special-election race to fill the seat made available after the death of Ted Kennedy. Mr. Brown, a Republican in an overwhelmingly Democratic state, beat Attorney General Martha Coakley in what amounted to a referendum on ObamaCare—though Mr. Brown's promised vote...