In case you didn't know, there is a special election today in Massachusetts with a lot riding on it. Republican state Sen. Scott Brown is attempting what will likely go down as the biggest upset in political history by beating Attorney General Martha Coakley in the special election for Ted Kennedy's Senate seat.

Even though there is a lot of talk about the national ramifications of the race – healthcare, the Democrats' 60-vote majority, etc. – there hasn't been a lot of discussion on how, exactly, Brown can win tonight. The path to statewide victory for a Republican in Massachusetts is a narrow one. So, here is what to watch for when the polls close tonight.

How Brown Wins
For Brown to win, he has to mimic how other Republicans have won in the Bay State. The most recent example of this is former Gov. Mitt Romney, with whom Brown now shares several advisors. In Romney's 2002 gubernatorial win, he carried Barnstable, Essex and Plymouth Counties by large margins. Brown will need big turnout in those areas.

Another way to look at the map is to follow I-495 in Massachusetts. This can be a Republican corridor. Brown must perform well in towns like Billerica, Marlborough, North Andover and Wrentham, Brown's home base. Another key bellwether for the GOP along I-495 is Chelsmford. Chelsmford is even more important in this race because it is nearly at the opposite end of the state from Wrentham, so it'll provide a good barometer for how well Brown is performing across the state.

How Coakley Wins
Don't buy the argument that this election comes down to independent, or unenrolled, voters. Yes, it is true that they make up half of the electorate in Massachusetts and Brown has targeted them. But this is a special election, which tend to be low turnout affairs. And it is snowing in Massachusetts. Democrats have a three-to-one registration advantage in the state. It is within the Democrats' control to win this race, they just have to get those voters to the polls.

Coakley needs to win big in Middlesex County – the state's most populous and where she was district attorney – and Suffolk County – where Boston is. In 2002, Romney carried Middlesex narrowly. If Coakley is lagging there, she is in big trouble. If Brown carries the county, he is heading to the Senate.

The Democrats also need to drive up the score in Suffolk. Boston is ruled by Mayor Tom Menino, who has probably the most effective political machine on the East Coast. There has been little written about whether Menino has put it into action for Coakley. Coakley also needs big turnout in Brookline, Cambridge and Somerville - big Democratic areas.

Other Places to Watch
Worcester County – Romney carried this county in 2002. (Don't confuse this with Worcester, the city, which is Democratic leaning.)

Fitchburg and Peabody – traditional bellwethers for Massachusetts in statewide elections.

For an in depth breakdown of voting behavior in Massachusetts, definitely check out this Partisan Voting Index of cities and towns on the Blue Mass Group.

A Final Thought…
Brown lives in the Bay State's 3rd Congressional District. If he doesn't win, he could challenge Rep. Jim McGovern (D), one of the most outspoken and liberal members of the congressional delegation, next year.

Before McGovern defeated him, the seat was held by Peter Blute, a Republican, for two terms. 

Jeremy P. Jacobs is the staff writer at Politics. He formerly covered Massachusetts politics for PolitickerMA.com. You can reach him at jjacobs@politicsmagazine.com