(CNN) The battle for the White House that has entertained, outraged and baffled Americans for the past year is about to get real.

With the Iowa caucuses and New Hampshire primary quickly approaching, a presidential race that has largely unfolded on television screens, debate stages, packed rallies and a slew of polls is now shifting into a serious test of political endurance.

"There is a very good possibility that the Republican primary will be decided by the end of March," Sen. Ted Cruz said during a New Year's Eve conference call , putting supporters of his fast-rising campaign on notice for a "90-day sprint to get the job done."

New Jersey Gov. Chris Christie, a Republican presidential candidate, poses for a photo with a visitor after speaking at a coffee house in Muscatine, Iowa, on Tuesday, December 29.

New Jersey Gov. Chris Christie, a Republican presidential candidate, poses for a photo with a visitor after speaking at a coffee house in Muscatine, Iowa, on Tuesday, December 29.

Chicago Mayor Rahm Emanuel speaks during a news conference at City Hall on Wednesday, December 30. Emanuel returned home early from a family trip to address the recent police shootings that killed Bettie Jones and Quintonio LeGrier.

President Barack Obama is seen on a television screen congratulating people in Miami who had just been sworn in as naturalized U.S. citizens on Tuesday, December 29.

President Barack Obama is seen on a television screen congratulating people in Miami who had just been sworn in as naturalized U.S. citizens on Tuesday, December 29.

A light casts a shadow of U.S. Sen. Bernie Sanders as the presidential candidate speaks at a town-hall event in Muscatine, Iowa, on Tuesday, December 29.

A light casts a shadow of U.S. Sen. Bernie Sanders as the presidential candidate speaks at a town-hall event in Muscatine, Iowa, on Tuesday, December 29.

U.S. Sen. Marco Rubio, a Republican presidential candidate, playfully throws a snowball at his wife, Jeanette, as they take their two boys sledding in Des Moines, Iowa, on Wednesday, December 30.

U.S. Sen. Marco Rubio, a Republican presidential candidate, playfully throws a snowball at his wife, Jeanette, as they take their two boys sledding in Des Moines, Iowa, on Wednesday, December 30.

Shamans in Lima, Peru, hold a poster of U.S. presidential candidate Donald Trump as they perform a ritual of predictions on Tuesday, December 29. The ritual is an end-of-year tradition for the shamans, who called for world peace and wished good luck for the upcoming elections in the United States and Peru.

Shamans in Lima, Peru, hold a poster of U.S. presidential candidate Donald Trump as they perform a ritual of predictions on Tuesday, December 29. The ritual is an end-of-year tradition for the shamans, who called for world peace and wished good luck for the upcoming elections in the United States and Peru.

The U.S. Capitol is seen behind the Washington Monument as day breaks on Thursday, December 31.

The U.S. Capitol is seen behind the Washington Monument as day breaks on Thursday, December 31.

Heading into the first contests of the 2016 campaign season, Cruz tops GOP polls in Iowa, billionaire Donald Trump is on top in New Hampshire and nationally, and Democrat Hillary Clinton hopes to quell a stronger-than-expected challenge in both states from Sen. Bernie Sanders.

But with many voters only now seriously sizing up the race for the first time, polls are sure to shift and only the results in the early voting states themselves will answer some of the burning questions that have built up over the past year:

Will Trump's convention-busting campaign really change the way presidential nominations are won? The reality show star made a name by firing people, but how will he react if voters decide to show him the door? Can Cruz expand his base from evangelicals and tea party conservatives to convince Republicans he has a real shot against Clinton? And will the GOP establishment coalesce behind a single candidate to take on Trump? Florida Sen. Marco Rubio, New Jersey Gov. Chris Christie, ex-Florida Gov. Jeb Bush and Ohio Gov. John Kasich are all vying for what may be a single ticket to later primaries.

And Clinton faces a nervous run-in to Iowa and New Hampshire because any early losses to Sanders could awaken the ghosts of 2008 when voters blocked her "inevitable" candidacy.

Here is a guide to the state of play in the early states and the lowdown on which candidacies are thriving and which could fade before the later sweep of Southern and larger states that could decide the nomination.

Iowa

Iowa looks like Cruz country, 27 days before caucus-goers traipse through a chilly night to churches, sports clubs and school gyms to select a nominee. He has consolidated two crucial blocs of the state's GOP coalition -- evangelical and tea party voters, and victory in Iowa would leave the well-financed Cruz strongly positioned for the Southern state delegate harvest to come.

Once a long shot, Cruz has prospered from tight self-discipline on the stump, an authentic conservative message and a reputation for throwing wrenches in the works in Washington that antagonize party elites but delight the restive grass roots.

"He had a very narrow window -- now he has got one of the best shots to win it," said Republican strategist Ford O'Connell.

In the Des Moines Register/Bloomberg Politics poll in early December, Cruz led Trump by 10 points, with one-time Iowa front-runner Ben Carson a further 8 points back in third.

But voters are still making up their minds amid a torrent of attack ads and candidate bus tours, town hall meetings and rallies.

"I like Carly (Fiorina), I like Rubio, I don't like some of things Trump says, but I like some of things he says that need to be said," said Iowa voter Marge Delzell, 75, of Sioux City.

"We were originally for Carson, but I don't know that he can beat Hillary," she said, underlining the desire of many voters for a slayer of the likely Democratic nominee.

The Hawkeye State is also a big test for Trump.

At first blush, the billionaire real estate mogul's brand of secular, rude, nasty politics seems a poor fit for a state where conservatives pride civility, faith and ideological purity. But his experience inside business and outside politics is connecting with Republicans who despair at their party's leadership. Some 39% of GOP voters in the Register poll said they were looking for a government outsider who has handled complex issues and managed teams.

It's been easy to forget during his long reign atop the polls, but Iowa will be the first election in which Trump has ever stood. So the caucuses will help answer a fundamental 2016 question: Will his band of angry followers who disdain the political system actually show up and vote? And how will Trump respond if he loses? Would defeat undercut the rationale of his campaign -- that he is a perpetual winner -- and melt his White House hopes before he leaves the snows of Iowa for the snows of New Hampshire?

Amid reports that he has begun intensively building a traditional get-out-the-vote operation in Iowa and elsewhere, Trump seems to understand the stakes in the early voting states and claims he will start spending $2 million per week.

"Don't sit back and say, 'Oh, Trump is going to do well.' The more we can win by, you know, the more power we have in a sense, because it is like a mandate. But you have got to go out and vote," he told supporters in South Carolina last week.

Rubio, who is trying to prove he can win over conservatives, last week shipped in South Carolina Rep. Trey Gowdy, a hero to the right after his October interrogation of Clinton over Benghazi. The senator's work on immigration has cost him some support with the right, and Gowdy's backing is designed to give him a character witness for Rubio's own patriotic, aspirational, promise to renew the American Dream.

"It's ... OK to be hopeful as you are delivering the message of conservativism. You don't have to be angry, and you don't have to be upset; you can be hopeful," Gowdy said at a town hall meeting in Newton, Iowa.

As evangelical voters go for Cruz and Carson, Iowa could be where we say goodbye to long-shots such as 2012 caucuses winner Rick Santorum and 2008 victor Mike Huckabee, who were unable to build support there this time around.

Among Democrats in Iowa, Clinton is leading, atop a humming state political network she built after learning the lessons of a third-place finish in 2008. Troubles here could raise questions about her campaign and whether she can connect with the Democratic Party's liberal wing. Her team is taking nothing for granted since she trails Sanders in the next contest in New Hampshire. However, victory in Iowa, twinned with a likely rout of Sanders in South Carolina, could all but guarantee her the nomination.

"The stakes are very high for her here. She has got to win. If she does, fine. If she doesn't, then it becomes a much different game in New Hampshire," said David Yepsen, a veteran journalist who covered the caucuses for the Des Moines Register for more than three decades. "It's really Sanders' only hope to get something going is to upend her here."

New Hampshire

The February 9 New Hampshire primary could spread carnage through the ranks of moderate and establishment Republicans.

But there's definitely hope for at least one of the slumbering big beasts of the GOP to make a mark. That's because Granite State voters have an independent streak and often pride themselves in anointing an alternative to the winner in Iowa and are more fixated on economic and national security issues than the social themes that dominate the caucuses.

In theory, that could boost candidates who can appeal to a more centrist audience such as Bush, Christie, Kasich and those who can straddle both conservative and moderate camps such as Rubio.

But Trump, a huge, disruptive force, has trashed assumptions in New Hampshire and the proliferation of candidates battling for the same audience in the bloated Republican field has also made it so far impossible for one to emerge as the true alternative to the real estate mogul and to catch him in the polls.

"I don't like Trump. I wish he wasn't the front-runner, I hope somebody, anybody else would knock him out," said Republican voter Peter Smiglis from Windham, New Hampshire. Smiglis sees Bush as too weak and thinks Rubio and Christie have waffled on illegal immigration, the issue that has fired up the conservative base.

A Boston Herald Franklin Pierce University poll in mid-December showed Trump leading at 26% and reveals the demolition derby unfolding in his wake with Rubio and Cruz at 12%, Christie at 11%, Bush at 10% and Kasich 2 points back.

Bush is fighting for his life, living out of a suitcase in the state, relocating most of his campaign staff to New Hampshire and unleashing a newly aggressive stump persona and multimillion dollar ad barrage against Trump and his rivals for the establishment lane. For Christie, showing signs of life, and Kasich, New Hampshire also represents a make-or-break moment. Rubio could be on the skids, meanwhile, if he bombs in Iowa and can't make a mark in New Hampshire.

Trump, though, would do well to take nothing for granted. Granite State voters are notorious for making up their minds late.

"I'll decide probably when I walk into the voting booth," said voter Heidi Milbrand, at a recent Christie event in New London.

State GOP official Ray Tweedie, meanwhile, says voters are "drilling into the weeds" on issues to separate the candidates and predicts a traumatic Tuesday, February 9, for a number of top contenders. "I think there will be a massive winnowing down of the field come Wednesday morning," he said.

Democrats also face a difficult choice. New Hampshire lies next to the home patch of Sanders in Vermont, and its mainly white, liberal and more elderly Democrats mirror the Vermont Senator's coalition.

But the state also made both Bill and Hillary Clinton comeback kids in 1992 and 2008, and the former first couple maintain a deep political network there.

Sanders has consistently led Clinton, but should she mount a come-from-behind win, the obituaries will be rolled out for his 2016 campaign.

South Carolina

South Carolina has often been the tie-breaker, settling the fight between Iowa and New Hampshire. Republican voters in the Palmetto State have backed the eventual nominee in every primary race since 1980, with the exception of 2012 when Newt Gingrich won.

"There is a sense here that people want to get that reputation back," said David Woodard, a political science professor at Clemson University, of the February 20 primary.

The most recent Winthrop University poll shows Trump out front, with 24% and Ted Cruz with 16% and Carson coming in third with 14% support about likely GOP voters. But there is also some good news for Rubio, at 11%, and Bush, with 9% support, compared with his roughly 3 or 4% backing nationally.

And Rubio's endorsement from Gowdy, who represents voter rich Greenville in the Upstate, could be significant. But Gingrich's 2012 win, which came after consecutive losses, demonstrated how debate performances and an anti-elite message can resonate with voters.

Culturally, Cruz, a Texan who wears black ostrich cowboy boots and quotes the Bible with ease, might be the best fit in South Carolina, where nearly 60% of likely GOP voters described themselves as "evangelical" or "born again" in a recent Winthrop poll.

Rick Vasso, 61, originally favored Carson but now thinks he lacks the leadership qualities needed of a president. He has narrowed his choice down to a candidate he initially thought was too conservative to win the nomination, but he has since seen rise steadily in the polls.

"If the primary were tomorrow, I would vote for Ted Cruz," Vasso, of Greenville, said. "He has a good chance to beat Hillary Clinton." But Rubio wouldn't be a bad second choice, Vasso said, though he prefers Cruz's stance on illegal immigration.

"They would be a great one-two punch," Vasso said. "If one wins, then the other is on the ticket as the vice presidential pick, that could set up the party for 16 years."

As for the Democrats, the test of Clinton's Southern firewall begins here.

Roughly half of the voters in the February 27 primary will be African-American, and Clinton has outpaced Sanders by wide margins among that demographic -- a recent Fox News poll shows her with 80% backing among African-Americans to Sanders' 11%. A strong showing by Sanders could upend Clinton's strategy, and set up a tougher fight in the Southern primaries to come.

Nevada and beyond

Coming out of South Carolina, where 50 delegates are at stake, the remaining candidates face a test of organization in Nevada and a test of their conservative credentials in a slew of mostly Southern states on March 1 when nearly 600 delegates will be up for grabs. The significance of the GOP Nevada caucuses -- the first in the West contest -- on February 23 depends greatly on results of the other previous contests. As with other caucus contests, polling has been patchy.

The latest CNN/ORC poll for Nevada in October had Trump on 38% with a double figure lead over Carson before his nationwide poll slump. If there is any candidate who might have an edge in Nevada, it's Rubio.

The Florida senator lived in Las Vegas until middle school and has been heavily courting Mormons -- as a child, Rubio and his family belonged to the Church of Jesus Christ of Latter-day Saints for about three years before returning to Catholicism.

Cruz is also fighting for the support of Mormons, who could make up a quarter of the GOP electorate. Nevada, where 30 proportionally allocated delegates at stake, could offer the morale boost of a victory for Rubio or another win for Cruz. Clinton, who won Nevada in 2008, is heavily favored in the February 20 Democratic caucuses thanks to her popularity among Latino voters.

Still, there is no bigger day than "Super Duper Tuesday," when 13 states will hold contests, the most for a single day. Clinton is betting that a sweep of Southern states Alabama, Arkansas, Georgia, Tennessee, Texas, and Virginia will propel her to the nomination. Sanders will likely win his home state and do well in Massachusetts and states with similar demographics.

On the GOP side, Cruz has a similar strategy to Clinton's, betting that evangelical voters in states such as Georgia and Arkansas will give him a boost.

"What happens in SEC primary is dependent on what happens in February, is it a big day, sure, a lot of delegates at stake, they are proportional states," said Justin Putnam, who runs the blog FrontloadingHQ. "It's a sequential process, and we will see who is viable and left standing."

While contests in deep red states will dominate, states such as Virginia, Vermont and Massachusetts also hold contests. The winner of those states could lay claim to the establishment title, arguing that wins there prove electability in the general election.

But because of the fact that many states award their delegates on a proportional, even the SEC primary may not crown a nominee. So though a candidate such as Cruz looks strong, he may still not clinch the nomination.

"If it's Cruz wins, Cruz wins, Cruz wins, that's a big momentum. It will look and sound powerful, despite the delegate numbers, Cruz is clearly making it his strategy to win in the South, and it's smart from a perception standpoint," said Angie Maxwell, an assistant professor at the University of Arkansas who writes about politics in the South.

"In terms of delegate counts, it doesn't do as much as it would have done had they been winner take all primaries, but the headlines could be a game-changer." said Maxwell.