Public Policy Polling rated the Massachusetts Senate race a tossup yesterday after releasing a poll showing Republican state Senator Scott Brown leading state Attorney General Martha Coakley 48-47.

Special elections are notoriously hard to decipher, and PPP got the NY-23 race wrong by over 20 points from their final pre-election poll. Like this one, that showed a heavy Republican turnout on Election Day and a disinterested Democratic electorate, buoyed by a candidate who caught fire among the far right. And this race is looking just as difficult to poll. Today’s Boston Globe poll, which is a bit older but not ancient, has Coakley up by 15 points, by 17 when you add leaners.

How to explain for the discrepancy? Really the formulation of a “likely voter” is the key. And PPP sees the likely voter as a Republican or GOP-leaning independent to a far greater degree than the Globe poll. From the analysis by PPP’s Tom Jensen:

Republicans are considerably more enthusiastic about turning out to vote than Democrats are. 66% of GOP voters say they are ‘very excited’ about casting their votes, while only 48% of Democrats express that sentiment- and that’s among the Democrats who are planning to vote in contrast to the many who are apparently not planning to do so at this point.

Brown has eye popping numbers with independents, sporting a 70/16 favorability rating with them and holding a 63-31 lead in the horse race with Coakley. Health care may be hurting Democratic fortunes with that group, as only 27% of independents express support for Obama’s plan with 59% opposed.

Even in the Globe poll, among voters “extremely interested” in the race, Coakley and Brown are tied, 47-47.

Despite today’s better numbers, national Democrats are starting to view this race as a wake-up call. The DSCC sent out a red alert to their list, and Organizing For America did as well, with a link to a virtual phone bank for Martha Coakley.

If Brown were to defeat Coakley, there would be 59 Senators left caucusing with Democrats. With Republicans virtually unanimous in opposition to the President and the Congressional Democratic agenda, that would bring the legislative process to a near-standstill, and would practically eliminate the possibility for passage of any health care legislation. Even if the House of Representatives could be cajoled into passing the Senate bill without changes, after seeing Democrats lose a Senate race in MASSACHUSETTS, surely some skittish members of the Democratic caucus would peel off the bill.