From Suffolk University:
A bellwether is an area of a state that closely mirrors a statewide electoral outcome using similar election types from previous elections and other data. Suffolk University’s bellwether model has been used since 2002 and is 95 percent accurate in predicting outcomes but is not designed to predict margin of victory. All bellwether analyses carry a margin of error similar to a statewide poll.
In Lake County, Romney led Obama 47 percent to 43 percent with Independent Richard Duncan receiving 4 percent and Stewart Alexander (Socialist Party) receiving 1 percent, while 2 percent were undecided and 4 percent refused a response. Romney led 49 percent to 44 percent among those planning to cast ballots and led 43 percent to 41 percent among those who had already voted. Duncan, an Ohioan listed on the presidential ballot, received most of his support from voters who have already cast ballots for him in Lake County, causing neither major candidate to reach a decisive 50 percent there. …
Two New Hampshire towns, Epping and Milford, have mirrored the statewide New Hampshire vote in four out of four presidential elections going back to 1996. In Milford, Romney led Obama 51 percent to 46 percent and in Epping, a closer bellwether, Romney led Obama 49 percent to 47 percent.
All kinds of data at the link.