(AP) -- Only a week remains before the Iowa caucuses, the first contest in a tumultuous presidential campaign that has challenged long-held political assumptions.

The Republican race in Iowa is Donald Trump's or Texas Sen. Ted Cruz's to lose on the night of Feb. 1. While some party leaders are coming to grips with the prospect of Trump as the nominee, a group of more mainstream candidates is battling to beat expectations in Iowa, head into the New Hampshire primary on Feb. 9 with momentum and rise to challenge the front-running billionaire.

The Democratic race has evolved into a surprisingly heated contest between Hillary Clinton and Vermont Sen. Bernie Sanders, the self-described democratic socialist who has energized young voters and liberals. Sanders' late surge has revived memories of Clinton's surprising loss to then-Illinois Sen. Barack Obama in Iowa in 2008.

Even as Iowa lays down the first marker in the 2016 race, more potential uncertainty looms. Former New York Mayor Michael Bloomberg had jolted the race yet again by starting to explore a third-party run, particularly if Trump and Sanders are the nominees.

A few of the unknowns that voters in Iowa will answer caucus night:

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CAN ANYONE OR ANYTHING STOP TRUMP?

Trump has done and said so many things that would have ended the campaign of just about anyone else. Even he's amazed at his apparent inability to commit a political error.

"I could stand in the middle of Fifth Avenue and shoot somebody, and I wouldn't lose any voters, OK?" Trump said Saturday in Iowa. "It's, like, incredible."

Many professional Republicans distain Trump and worry about his long-term effect on the party's ability to win over general election voters. Trump has topped most preference polls for months, and the waiting for his star to fade long ago gave way to bafflement.

Yet Trump's success so far is based almost entirely on those polls. He has picked up Sarah Palin's endorsement and has the tacit support of several evangelical leaders in Iowa. Still, not a single voter has weighed in and tested whether Trump actually is unbound by political gravity.

True to form, Trump isn't shy about predicting success. His team may heed to a policy of "radio silence" when it comes to discussing the work to get out the vote, but the candidate continues to raise expectations, telling reporters and packed rallies that he will outperform the polls.

Prescient optimism or the first mistake of an unexpectedly flawless campaign?

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DID TED CRUZ PEAK TOO SOON?

Cruz has perhaps the clearest path to the nomination. As the new year dawned, he appeared poised to unite a fractured conservative base and become the leading force as the campaign focus shifted to South Carolina in mid-February and across the South in March.

That still might happen. But his momentum has stalled as high-profile conservatives defected to Trump and Cruz's rise drew stiff opposition from mainstream Republicans. Sen. Richard Burr, R-N.C., told supporters at a recent fundraiser that he would vote for Sanders, a self-described democratic socialist, over Cruz.

Iowa ought to be a place where Cruz's appeal among Christian conservatives gives him a distinct advantage. But Republicans who dislike Trump as much as they distain Cruz -- and many find both unacceptable -- now wish for a Trump victory in Iowa.

Cruz has a well-tended network of grassroots conservative supporters across the state. Can it keep him on top?

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DO CLINTON'S ATTACKS ON SANDERS SUCCEED?

For months, Clinton spoke relatively warmly about Sanders. The fear was that any harsh critique of the liberal senator who was pushing a "political revolution" might alienate his supporters, so important to the Democratic nominee in the general election.

As Sanders' standing in preference polls has risen, to the point where some show him ahead in Iowa, so has Clinton's rancor. In the past week, Clinton went after Sanders' foreign policy credentials, his health care proposal and his plans to crack down on Wall Street. She questioned his electability and cast him as a flip-flopper on gun control.

Clinton's allies point out that an ad released by the Sanders campaign, an uplifting montage of Americana set to a Simon and Garfunkel tune, featured mainly white people, and they are casting it as a sign of the senator's disrespect for minorities.

Clinton's campaign says Sanders was overshadowed by the raucous Republican field and subjected to little scrutiny earlier in the race. Now that his views are getting more attention, the Clinton team says, his support will begin to fall. But Sanders has compared Clinton's efforts to those in the 2008 race, when she knocked Obama's proposals and experience. "People of Iowa saw through those attacks then and they're going to see through those attacks again," he said Saturday.

If he's right, will Clinton endure another Iowa backlash, again fueling a rival's insurgent campaign?

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CAN MARCO RUBIO PULL OFF HIS LATE IOWA PUSH?

For months, the Florida senator ran what many observers saw as a national campaign. Now, Rubio is making a late, aggressive push in Iowa. His team argues that more Iowans will have seen him, and many will have met him, than have seen or met any other candidate.

In Bettendorf last week, for example, a room set for roughly 300 at a country club was full, with more than 200 people left standing by the time Rubio arrived. He stayed to shake hands with hundreds, which slowed the exit flow and allowed aides to sign up supporters and volunteers.

Yet only recently has Rubio visited GOP strongholds such as Sioux Center in northwest Iowa. He has not gone to Denison, a GOP hub in Crawford County.

Rubio's Iowa staff doesn't compare with Cruz's. But Rubio benefits from Conservative Solutions, a super PAC with no official ties to Rubio's campaign. The group is identifying voters by phone, online and social media and using the connections to help turn out Rubio supporters.

Rubio is seen as the favorite in some GOP quarters to emerge as the alternative to Trump and Cruz. He got a boost Saturday night with the endorsement of The Des Moines Register.

Does he have enough on the ground in Iowa to pull out a strong finish and carry that bounce into New Hampshire?

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WHO SHOWS UP TO VOTE ON CAUCUS NIGHT?

Party loyalists tend to dominate the Iowa caucuses, but an influx of independent and young voters can create a winning formula. Sanders and Trump hope to expand the size of the caucus by generating a big turnout among both groups.

Only Republicans and Democrats can participate in the caucuses, but the state allows same-day registration for independents. In 2008, the last time Democrats had a contested caucus, about 20 percent were independents, and Obama won about 40 percent of those caucus-goers. Young voters accounted for about 20 percent of the caucus that year, and Obama won more than half.

In 2012, entrance polls showed that nearly one-quarter of Republican caucus-goers were independents, and Ron Paul took more than 40 percent. The then-Texas congressman also succeeded among caucus-goers 29 and under, capturing about half of that group.

While Paul's son, Kentucky Sen. Rand Paul, is in the GOP race this year, Trump has the most riding on these voters.

"More than any other candidate, Trump's success will hinge on his ability to expand the Iowa caucus electorate by turning out independents and first-time caucus attendees," said former Iowa GOP Chairman Matt Strawn.

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Associated Press writers Julie Pace, Jill Colvin, Steve Peoples, Lisa Lerer, Thomas Beaumont and Ken Thomas contributed to this report.