Our NBC News/Marist poll of Wisconsin shows Democrat Tony Evers leading Republican Gov. Scott Walker by 10 points in a head-to-head contest, and by 8 points in an expanded ballot that includes the Libertarian and Green Party candidates.

A day earlier, however, a Marquette Law School poll had Walker ahead by 1 point. (The polls were on the same page showing Sen. Tammy Baldwin, D-Wis., ahead by double digits.)

So what explains the difference between the two polls? We asked Marist pollster Barbara Carvalho. Here's her answer:

1. The polls use different likely-voter models: Marquette's model is based exclusively on voters certain to vote; Marist's uses a probability model based on interest, chance of vote and past vote.

2. Voter ID: Marquette's sample among likely voters was R+3; Marist's was D+1.

One other observation from Carvalho:

"Finally, one number to look at in our poll is the 80% of Walker supporters who are strongly committed to him (an extraordinarily high number). This intensity would be magnified in a cutoff 'certain to vote' likely voter model which Marquette uses. Of course, it is something to watch and a reason not to count Walker out."

** Correction: An earlier version to this story noted that Marquette uses a voter list and not random digit dialing. But that was not correct. It uses random digit dialing.