The distribution of the vote shifted for Martha Coakley but not for Scott Brown last night. That was the key to Brown's win.
The top left of the chart above shows the distribution of Coakley's vote compared to Brown's. Brown's better total shifts his distribution clearly to the right. That's not interesting. But the bottom row is very interesting. The bottom left panel compares Coakley (dark blue) with Obama's light blue distribution in 2008. She's well to the left, doing worse. Of course you'd expect drop-off from a presidential to a special election. But the bottom right panel is amazing. Brown's distribution almost exactly duplicates McCain's. In a January special election, Brown's vote is a clone of McCain's in a presidential contest. That is amazing.
Here is another way to look at it. Plot last night's vote by town against their party's candidate in 2008.
Brown's votes are almost exactly on the 45-degree line showing equality between 2008 and 2010 vote totals by town. But not so the blue dots, which are all, yes every single one, well below the diagonal. Brown's total actually slightly improved upon McCain's. Coakley's total was just 56% of Obama's total.
The chart is powerful but the logarithmic scale makes the two clusters of points appear closer than they "really" are. Let's plot Coakley as a percent of Obama vote against Brown as a percent of McCain for a more compelling view.
Wow. The imbalance of performance is stark. Coakley's BEST town gave her 80% of Obama's vote. That's equal to Brown's WORST towns. Even the towns Coakley won were places she was dramatically underperforming Obama. And there are no pockets of strength visible here. Brown was doing over 100% except in a few blue towns but he even outperformed McCain in a number of towns that went for Coakley.
Of course this doesn't mean that Brown got exactly McCain's voters, since lots of individual switching could add up to these totals. But in the aggregate, Mass. in 2010 looks exactly like it did in 2008 on the Rep side. On the Dem side, a whole lot fewer voters.