So with the new Star Tribune poll out showing DFLer Mark Dayton with a 9-point lead over Republican Tom Emmer, it’s the right’s turn to howl over alleged bias. In the spirit of Dems accusing Rasmussen Reports of being a Republican house organ, Mitch Berg at the True North blog dubs the Strib results “The DFL morale-booster”:

I’ll remind you that if the Minnesota poll were accurate, we’d be referring to Governor Humphrey (the poll showed Moe with a strong lead over Coleman, with Ventura well out of the running), Senator Mondale (who had a five point lead in the MN Poll on the eve of the ’02 election), Governor Moe (to whom the MNPoll gave a slim lead, while significantly overpolling IP candidate Tim Penny in ’02), Governor Hatch (yep, slated to win in ’06)…

This got me wondering just how the Strib’s poll has done more recently — and versus other polls that the right regards more highly (Rasmussen, SurveyUSA). As some of you may know, the Strib changed pollsters in 2007, when it let go in-house polling director Rob Daves, replacing him with Princeton Survey Research Associates for the 2008 cycle.

An important caveat about pre-election polling: missing the final margin doesn’t necessarily mean a pollster is wrong. Sentiment can swing in the voting booth, after polling ends. (This is why pollsters refer to their results as a “snapshot in time.”) Also, any poll has margin of sampling error. The trick is to see patterns — the so-called “house effect” toward a particular party, and whether results are consistent outliers.

Let’s begin with Daves’ last cycle, the 2006 election.

Mitch rakishly references “Gov. Hatch.” Here are the three major pollsters’ final November results, via Real Clear Politics’ roundups:
Star Tribune, Hatch +3
Rasmussen, Hatch +2
SurveyUSA, tie

Survey USA won the crown in a race Pawlenty eeked out by 1. But no poll — including the Humphrey Institute, Mason-Dixon, Zogby Interactive, or St. Cloud State — had Pawlenty ahead in November … or October.

By the way, Daves and the Strib finished closest to the pin on that year’s Senate race, when Amy Klobuchar bested Mark Kennedy by 20:
Star Tribune, Klobuchar +21
Rasmussen, Klobuchar +16
SurveyUSA, Klobuchar +14

OK, what about the real litmus test: the 2008 Senate race between Franken and Coleman? I’ll skip the drama and say no one predicted the virtual tie:
Star Tribune, Franken +4
Rasmussen, Coleman +4
SurveyUSA, Coleman +5

Everyone missed by about the same amount.

In this three-race survey, the Strib was the only poll to give the DFLer the lead all three times, lending credence to Republican claims of a pro-Democrat “house effect.” The Dayton result — which for now is an outlier — is another brick in that wall.

However, if you’re arguing a poll equals a prediction (which again, isn’t quite right), the Strib picked two winners, SUSA two (we’ll give ’em the TPaw tie) and Rasmussen only the AKlo blowout.

Even allowing for GOP mewling that Franken stole the 2008 election, it seems clear that the three polls have circled the final result roughly equally. I’d also note that, at least from 2006 on, if you’re comparing the final polls to the eventual outcome, SUSA’s house effect is as Republican as the Strib’s is Democratic.

As I’ve noted in several columns this month, the Strib’s 2010 polling now include cellphone-only voters, a potentially significant methological difference with Rasmussen, SUSA, and the Humphrey Institute/MPR poll. (The Strib and HHH use live operators; SUSA and Rasmussen are so-called “robo-polls” using recorded questions and touch-tone answers.)

A potentially bigger difference: how each pollster screens for likely general-election voters. I’m surveying the major pollsters on their “likely voter screens” and will let you know after I hear back from everyone.