WASHINGTON — At this point four years ago, national polls taken in the run-up to presidential primaries said to prepare for a general election matchup between Republican Rudy Giuliani and Democrat Hillary Rodham Clinton.
In 2004, Howard Dean was coasting to the Democratic nomination to face President George W. Bush.
Neither pairing materialized.
As the past has proved, countrywide polls are hardly crystal balls, particularly for primary elections that are won state by state. National surveys give insights into what people are thinking, and why, rather than predicting how they will 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 Republican presidential caucuses, will win. Also, there's no certainty over whether Rick Santorum, Rick Perry and Newt Gingrich will remain at the back of the pack.
On top of that, it's hard to find true primary voters to survey because so few people vote. In 2008, a record 120,000 Republicans voted in the Iowa caucuses, but that was only 2 percent of Iowa's adults.
Consider the 2007 primary polls. As Iowa approached, most national polls said Clinton and Giuliani were the front-runners. The final Associated Press-Ipsos poll that year put Clinton solidly ahead of Barack Obama, while on the GOP side, Giuliani held a comfortable edge over the rest of the field.
But Obama won Iowa for the Democrats, as did Mike Huckabee for the Republicans.
The national polling didn't say exactly how people would vote in 2008, and 2011 polling won't reveal who will win in 2012.
But countrywide surveys do provide context and help to explain the shifting results as the primaries roll on.
In the 2008 nomination contests, polls indicated that voters were focused on the war in Iraq and deeply dissatisfied with the Bush presidency and the country's direction. Those themes said more about why Obama and Republican John McCain won their party's nomination than the head-to-head matchups did.
This year, polls have consistently shown two dominant themes in the GOP race:
—A tepid response to the GOP field among Republican voters.
Earlier this month, an AP-GfK poll found that amid Gingrich's rise, Republican dissatisfaction with the lineup of candidates also rose. The wild swings among the anyone-but-Romney crowd have lifted nearly all of the candidates at some point this year, but none has fit the bill exactly.
Republicans don't actively dislike Romney, with 73 percent saying he's a strong leader and 81 percent calling him likable. But his best showing in any poll this year remains around 30 percent, and no other candidate has pulled a strong showing among the remaining 70 percent of the party.
—A deep anger among Republicans toward Obama.
This sentiment has buoyed Romney through the rise and fall of other candidates. Poll after poll reinforces his status as the candidate who runs most competitively with Obama and is seen as the most electable. Republicans sure would like to see Obama voted out of office. In the latest AP-GfK poll, 89 percent of Republicans said he deserved to be voted out, and three-quarters said they expected him to lose.
Later this year, the Republicans' battle for voters and delegates will end and a single Republican nominee will be left standing. But no one should expect today's polling to say who that will be.
Jennifer Agiesta is AP's deputy director of polling. She can be reached on Twitter at http://twitter.com/JennAgiesta .