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 indicaton.

The map below gives us an early, mischief-making 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. Green communities lean toward Patrick, and purple 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 results (click to enlarge) give us an idea of each man’s geographical base:

In terms of land area, Patrick’s base is in the western third of the state, but his biggest advantage in actual votes would probably come from Boston and other large cities. His primary percentage was more than 20 points ahead of Cahill in Boston, Newton, Cambridge, Springfield, and Somerville (in decreasing order of votes cast in the primary), among other communities.

Cahill’s geographical strength was centered on the South Shore, though he did very well in some Merrimack Valley towns. (One of his opponents was Boston City Councilor Steve Murphy, who concentrated not only on Boston but on the state’s Gateway Cities.) Cahill did better than Patrick in his hometown of Quincy, plus Plymouth, Lowell, Weymouth, and Methuen.

If Patrick and Cahill maintained their respective geographic bases, the results of a primary may come down to cities and towns where neither candidate have done especially well (and a smaller number of communities where both have done well). The biggest cities where both Patrick and Cahill underperformed their state averages were New Bedford, Fall River, Medford, Peabody, and Lynn. The biggest places where both finished ahead of their state averages were Milton (Patrick’s hometown), Natick, and Randolph.

UPDATE: Just to identify some of the geographical outliers, that island of Patrick Green in the sea of Cahill Purple is the town of Sharon. And the corridor of Patrick communities heading northwest from Boston stops at Lancaster before hitting a wall with the Cahill city of Leominster. And heading straight north from Boston, strongly pro-Patrick territory extends to Winchester before being cut off at the pass by the Cahill communities of Woburn and Stoneham.