Iowa, it seems, is still undecided.

Fifteen days before that state’s caucuses formally begin the Republican presidential race, even longtime observers of Iowa politics acknowledge that they have never seen this level of uncertainty in the electorate.

“There is always some segment of the caucus electorate that is undecided going into caucus night, but the fact that over half of likely Iowa caucus attendees are telling pollsters they could change their mind in the final two weeks on which candidate to support is unprecedented,” said Iowa Republican Party Chairman Matt Strawn.

Said Dave Roederer, who was an adviser to President George W. Bush and Arizona Sen. John McCain in Iowa: “This may be one of those Christmases when you really don’t know what’s in the package.”

Polling affirms this unsettled state of affairs in Iowa. Although former House speaker Newt Gingrich has moved into a clear lead in the state over the past several weeks, about half — 52 percent — of likely caucus-goers said they could change their minds about a candidate, according to a Washington Post-ABC News survey.

Although Iowa has rarely chosen either party’s nominee — remember President Huckabee? — it has long served to winnow the field. For candidates playing in the state, which in this race means everyone but former Utah governor Jon Huntsman Jr., finishing in the top two (or, at worst, top three) is a necessity as they try to build momentum heading into New Hampshire (Jan. 10), South Carolina (Jan. 21) and Florida (Jan. 31).

Given Iowa’s winnowing power, playing the expectations game matters — a lot. Here’s a look at the Iowa expectations for the six candidates who are actively competing in the state — ranked in order of their likelihood of winning.

Newt Gingrich: There are signs that Gingrich may have peaked slightly too soon in the Hawkeye State. Not only is he organizationally behind, he is also being heavily outspent by his rivals — all of whom are attacking him — on television. That means that the average Iowa voter is seeing a whole lot of negative information about Gingrich and next to nothing positive about or from him. Gingrich seems to be hoping that his appeal to big ideas and not small attacks will carry him through. If it does, he’ll defy conventional political wisdom. Again. What he needs: Given his rise in Iowa and nationally, he must finish first or second.

Ron Paul: The congressman from Texas is the not-so-secret smart guy’s pick to win Iowa. Polling suggests he is running steadily behind Gingrich in the state but, unlike the former House speaker, Paul has put together an extensive political organization. There’s little question that his support in Iowa is deep; he, more than any other candidate, has a totally reliable base of caucus-goers. If that base is roughly the 11,800 votes he received in the 2008 Iowa caucuses, the question is how much wider he can grow his support. He’ll probably need to double it (at least) to finish in the top two. What he needs: A win would shock the political world. Second place would keep him in the game through New Hampshire.

Mitt Romney: The former Massachusetts governor’s arm’s-length campaign in Iowa has come to an end with the candidate and his super-PAC — Restore Our Future — going great guns in the final few weeks. It’s still hard to see Romney winning the caucuses because of a lingering trust issue with socially conservative voters. (Some suggest that’s because of his past flip-flop on abortion; others attribute it to his Mormon faith.) But, with the amount of money he and his allies are spending, it’s impossible for Romney to say that the caucus vote doesn’t matter. What he needs: Given his strength in New Hampshire, he could probably survive a third-place finish. But Romney would really like to see Paul, not Gingrich, wind up on top in the Hawkeye State.

Rick Perry: The left-for-dead Texas governor is experiencing a bit of an Iowa renaissance because of massive ad spending by the candidate and the super-PAC that is aligned with him. Perry has helped his cause over the past 10 days by outperforming expectations in two Iowa-based debates and by emphasizing his anti-Washington, outsider message. What he needs: It’s hard to see him winning, given the hole he dug for himself, but a second-place finish would be a big enough surprise to last him to South Carolina, where he needs to win.

Michele Bachmann: Four months ago, the congresswoman from Minnesota was the clear front-runner for the Iowa caucuses, having scored a victory in the Ames Straw Poll. After struggling through major staff turnover and a meandering message, she appears to have found herself again — emphasizing her consistent conservatism and her willingness to fight the tough fights. Bachmann’s problem is that she lacks the money to get that message out. What she needs: Without another obvious state where she can win, it’s hard to see her as a relevant factor if she can’t take first or second on Jan. 3.

Rick Santorum: If any candidate typifies just how un­or­tho­dox this Iowa campaign has been, it’s the former senator from Pennsylvania. He’s run the most traditional effort of anyone in the field, putting together a solid Iowa team and traveling to all 99 of the state’s counties. But he just hasn’t been able to find a spark. There are those in Iowa who argue that all of Santorum’s hard work will pay off in a better-than-expected finish on caucus night. He better hope they’re right. What he needs: A top-two finish is necessary but unlikely.