UPDATE: State Treasurer Tim Cahill is running for governor as an independent, not as a challenger to incumbent Gov. Deval Patrick in the Democratic primary. Still, this CW map from 2009 gives some idea of the geographic contours of this fall’s general election if Cahill manages to sneak past GOP nominee Charlie Baker to become Patrick’s main opponent.

what would the political geography look like if Treasurer Tim Cahill challenged incumbent Gov. Deval Patrick in next year’s Democratic primary? It would be Faneuil Hall and Tanglewood vs. Plymouth Rock and the Lowell Spinners, if past results are any indication.

The map below gives us an early look at the contest that may or may not happen. It’s a mash-up using the results of both men’s last competitive primaries. (Get the town-by-town numbers here.) Red communities leaned toward Patrick, and blue communities got on the “Tim for Treasurer” bandwagon. Patrick won the three-way 2006 gubernatorial primary with 49.7 percent of the vote; Cahill won the four-way 2002 treasurer’s primary with 35.8 percent of the vote. That Patrick finished 14 points ahead of Cahill doesn’t mean anything in itself, since the number of candidates and the dynamics were different in each primary, but the map below gives us an idea of each man’s geographical base.

Patrick dominated the western third of the state in 2006. Unfortunately for the governor, this is also the region that may react most negatively to his proposed hike in the state’s gas tax — especially if voters there feel that metro Boston will get most of the benefit of increased spending on mass transit and other transportation projects. But Patrick may be able to compensate for any drop-off in support west of Worcester by mining for votes in urban centers. He ran more than 20 points ahead of Cahill’s numbers in Boston, Newton, Cambridge, Springfield, and Somerville, for example.

Cahill’s strength centered on the South Shore, though he also did well in some Merrimack Valley towns. He was noticeably stronger than Patrick in his hometown of Quincy, plus Plymouth, Lowell, Weymouth, and Methuen.

If Patrick and Cahill maintain their respective bases, the results of a primary may come down to places where neither candidate has done especially well. The biggest cities where both underperformed their state averages were New Bedford and Fall River. So the South Coast, which has been faring the worst during the economic downtown, may get to flex some political muscle come 2010.