A billboard of Donald Trump in the backyard of George Davey's home Sunday in West Des Moines. (Charles Ommanney/The Washington Post)

As the eyes of the nation train on Iowa and voters here prepare to make their final choices, Democratic and Republican presidential candidates began Sunday racing to set expectations for Monday’s caucuses.

With Sen. Ted Cruz of Texas close on his tail, businessman Donald Trump continued Sunday morning to try to blunt Cruz’s appeal among Iowa’s powerful evangelical voters and the GOP’s hard-line conservatives.

“I think the big factor, if you look at Iowa and this one, is how well I’m doing with evangelicals. I’m leading with evangelicals. I’m leading with Tea Party,” he said in a telephone interview on ABC News’s This Week Sunday morning.

Trump also knocked Cruz, who has emerged as his primary rival in the Hawkeye State, for failing to work with his colleagues in the Senate.

“Nobody likes him… you can’t run a country that way,” Trump added. “It will be a total mess. It will be worse gridlock than you have right now.”

With Democratic candidates Hillary Clinton and Bernie Sanders virtually tied ahead of Monday's caucus, things are heating up on the ground in the Hawkeye State. Both candidates made a splash with large rallies over the weekend, but behind the scenes is where the real action is taking place. (Alice Li,Dalton Bennett/The Washington Post)

Front-runners Trump and Hillary Clinton both cling to tenuous leads over their principal rivals, Cruz and Sen. Bernie Sanders of Vermont on the eve of the caucuses. Meeting or exceeding expectations here could provide a slingshot of momentum into the New Hampshire primaries, which are just a week away.

Cruz, seeking to close the gap with Trump, worked to chip away at support for Sen. Marco Rubio of Florida, who may be benefitting from a last-minute surge of interest after a strong debate performance on Thursday.

“A vote for Marco is a vote for amnesty,” Cruz said on CNN’s State of the Union. “And I will tell you this, if we nominate a candidate who supports amnesty, who has the same position on amnesty as Hillary Clinton, we will lose.”

Rubio has struggled gain footing in Iowa, but he remains widely seen as the Republican establishment’s best hope to stop either Trump or Cruz. Rubio’s aides say that swelling crowds show that momentum could help him finish comfortably in third place. But he still faces the challenge of overcoming Cruz’s broadside against him on immigration policy, an issue that has in the past put Rubio out of step with much of the Republican base.

“The lie that his whole campaign is built on is that he’s the only conservative and everyone else is a sellout and a RINO [Republican In Name Only], and it’s absurd,” Rubio fired back at Cruz on CNN. “I think as people learn more about his record, they’ll realize that he really is very calculated. He’s always looking to take whatever position it takes to win votes or raise money.

“You know, we’re not going to beat Hillary Clinton with someone that will say or do anything to get elected,” he added.

Whether Clinton emerges victorious over Sanders seems headed to a battle between her superior organization and his groundswell of enthusiasm.

Clinton’s campaign remains confident that the grassroots juggernaut they built over more than eight months will pull them through, with an extra boost from former president Bill Clinton, who has traversed the state for his wife in recent days.

But hanging over Clinton is the specter of the ongoing investigation into her use of a private email server while she served as secretary of state. Just days before the Iowa caucuses, the revelation that 22 e-mails stored improperly on her private server may have contained classified information has reignited the controversy.

“It was not the best choice,” Clinton acknowledged in an appearance Sunday on This Week. “I wouldn’t be here talking to you about it. I’d be talking about what people in Iowa are talking to me about, about affordable health care and jobs and rising wages and all of the concerns that are on their minds.”

Appealing to Democrats weighing the potential political liability posed by Clinton’s controversies and his self-identification as a “democratic socialist,” Sanders volunteered that the email controversy has already proven to be a weight on Clinton’s general election prospects.

“Well, look, in terms of what people are going to get slapped with, look at the front pages today in terms of what Secretary Clinton is getting slapped with,” Sanders said. “You know as well as I do, it has to do with emails.”

The Sanders campaign – still hoping for an upset – has organized massive rallies in the state in recent days, full to the brim with young Democrats or first-time voters who are by their very nature less reliable voters, but who have been drawn to his populist message and pledges to eliminate the cost of higher education.

On both sides, the math and strategy of succeeding in the caucuses has made their data intelligence crucial.

The Sanders campaign, for example, has urged their college-aged supporters to “Go Home for Bernie,” an effort to move votes from densely populated urban college towns to less populated suburban and rural communities where Sanders could benefit more from marginal support.

In the last week, Cruz’s campaign laid out in meticulous detail exactly how many voters they must capture from Trump, Rubio and others to win here. Cruz is banking on a robust ground game he has rolled out over many months. Right now, the campaign said there are 12,000 volunteers on the ground in Iowa - but the Texas Republican stresses that he is running a national campaign.

“We don’t view any state as a must-win,” Cruz added. “I think we’re positioned to do very well in Iowa,” he said.

Jose DelReal in Des Moines, Sean Sullivan in Clive, Iowa, John Wagner in Iowa City and Katie Zezima in Sioux City contributed to this report.