FILE - In this Nov. 9, 2011, file photo, Republican presidential candidates former Pennsylvania Sen. Rick Santorum, left, Rep. Michele Bachmann, R-Minn., former House Speaker Newt Gingrich, former Massachusetts Gov. Mitt Romney, businessman Herman Cain, Texas Gov. Rick Perry, Rep. Ron Paul, R-Texas, and former Utah Gov. Jon Huntsman, pose before a Republican presidential debate at Oakland University in Auburn Hills, Mich. When it comes to education, the Republican field of presidential candidates has a unified stance to get the federal government out of schools, but disagree on methods. (AP Photo/Paul Sancya, File)

WASHINGTON (AP) -- At this point four years ago, national polls taken in the run-up to presidential primaries said to get ready for a faceoff between Rudy Giuliani and Hillary Rodham Clinton.

That match-up didn't materialize.

As the past has proved, countrywide polls are hardly crystal balls, particularly for primary elections that are won state by state. National surveys are more like thermometers, giving insights about what people think -- and why they think it -- rather than predicting how they'll vote.

State-level polling close to an election can be predictive, but even that is far from precise. Voters always can change their minds and attitudes shift quickly.

So there's no guarantee that Mitt Romney or Ron Paul, who are leading in Iowa polls just days before Tuesday's leadoff presidential caucuses, will win.