State of 2016 play: Questions only Iowa caucuses can answer

DES MOINES, Iowa -- 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 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.

With a week to go, a look at a few of the unknowns that voters in Iowa will answer caucus night.

Marco Rubio

Can 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. "It's, like, incredible."

Many professional Republicans disdain Trump and worry about his effect on the party's ability to win over general-election voters. Trump has topped most preference polls for months.

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 he will outperform the polls.

Did 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 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 disdain Cruz -- and many find both unacceptable -- now wish for a Trump victory in Iowa.

Did attacks on Sanders succeed?

For months, Clinton spoke relatively warmly about Sanders. The fear was 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 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 said 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 said, 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.

Can Rubio pull off a late 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 more Iowans will have seen him than 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 political action committee 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.