MESA, ARIZONA - One place where outcome of tonight's 20th - and potentially final - Republican debate may have the least influence is here in Arizona. According to ABC News reporting, more than 275,000 Arizonans have already voted absentee. All but a handful of those ballots are for the GOP primary (the Green party also holds a primary on January 28th).

If turnout is similar to 2008 - in which 541,000 voted in the GOP primary - it means that about half of likely Republican primary voters have already cast a ballot.

And that's good news for Mitt Romney. Rick Santorum may have the momentum, but with up to half the votes already cast, and Newt Gingrich still in the high double digits in the most recent polls, Santorum doesn't have a lot of room to grow.

One other ominous sign for Santorum. According to an NBC/Marist poll released today, Romney was leading Santorum by 16 points. That poll also showed that 53 percent of respondents had already cast their ballots - another indication that Romney already has a lot of votes already sitting in the bank.

That said, if Santorum has a strong showing with voters who cast their ballot on election day, it will confirm Romney's lack of mo-jo within the GOP base.

ABC's Chris Good contributed to this report.