Scott Brown vs. Martha Coakley: It's All About Enthusiasm

(AP/Steven Senne, Winslow Townson)

Yes, Massachusetts is a very "blue" state –- Obama won it by a lopsided 62 percent, and its Congressional delegation is entirely Democratic –- but we need only look back as far as New Jersey two months ago for an example of how that doesn't matter if enthusiasm shifts to the GOP.

Enough Democrats stayed home for Jon Corzine's re-election bid, while Independents and Republicans rallied in that usually-Democratic state. The Independents who did show up in New Jersey went overwhelmingly for Republican Chris Christie (60 percent - 30 percent); Obama had won Independents there in '08. The '09 New Jersey electorate was a little less proportionally Democratic than it had been in '08 (41 percent vs. 44 percent) and, as is often the case in off-year and special elections, smaller overall.

In Massachusetts, there are enough Republican-voting Independents to make this close or tilt it to Scott Brown (who's campaigning as an outsider, but also as the would-be 41st vote against the Democrats' health care bill) though that's particularly so if it is a lower- or lopsided-turnout election.

Consider: while only 12 percent of voters are registered as Republicans, 17 percent called themselves such in 2008, and John McCain got much more than that in total vote share, 36 percent. In doing so McCain drew 40 percent of Massachusetts' Independents. It wasn't enough for him, but it suggests that the numbers are there if they show up and Democrats don't, and of course more so if some Democrats back Brown.

Solid Democratic turnout for Coakley would be key for her mitigating any losses she does take among Independents. Watch to see if turnout approaches midterm levels -- which it could, given the spike in interest now.

The public polls in the Mass. race have shown a lot of variation, in part because it's difficult to poll a special election when turnout is such a wild card. Voters can vacillate on whether they'll show up on an unusual election date, and pollsters can differ in how they define a "likely voter" as they try to estimate the final size and composition of the electorate.

Statewide, there are three times as many registered Democrats as Republicans: 37 percent registered Democrats, or 1.5 million voters, compared to just 12 percent registered Republicans, or 490,000. The 2008 electorate was composed of 43 percent Democrats, 17 percent Republicans, and 40 percent Independents.

In terms of specific places, this may be won and lost in the suburbs to the north and immediate northeast outside Boston, a vote-rich collection of places where Mitt Romney did fairly well in winning the governorship in 2002.

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Anthony Salvanto is CBS News Elections Director.