US presidential candidates made a frenzied, final push Sunday to lock in Iowa voters on the eve of the first nominating contest of the 2016 election season.
Republican Donald Trump and Democrat Hillary Clinton both were leading in the polls, but not by enough to assure victory for either candidate in Monday's caucuses.
Armies of volunteers fanned out through the snow-swept Midwestern state, knocking on doors or manning phone banks to get out the vote, while candidates dominated the air waves.
Clinton, fearful of a repeat of 2008 when she was beaten to the punch in Iowa by an upstart Barack Obama, was leaving nothing to chance, stumping in the heartland this weekend as her main rival Senator Bernie Sanders did the same, seeking to deny her yet another shot at history.
A Bloomberg/Des Moines Register poll published late on Saturday -- the final one of the campaign -- put Clinton just ahead of Sanders, 45 to 42 percent, with a four percent margin of error.
The same poll gave Trump a bigger lead over his Republican rivals: The billionaire businessman had the support of 28 percent of voters, followed by Senator Ted Cruz at 23 percent, and Senator Marco Rubio with 15 percent.
"This race is as tight as can be," David Axelrod, President Obama's former political strategist, told The Des Moines Register.
Sanders has energised young Democrats with his denunciations of the "billionaire class" and his calls for a political revolution.
If the thousands of students who fill his rallies turn out on Monday they could produce an upset.
"If the turnout is high," Sanders said on CNN's "State of the Union, "I think we've got a real shot to win this."
Trump and his nearest rival Ted Cruz meanwhile made appeals to Iowa's evangelical conservatives so important in the first-in-the-nation contest.
And long-shot hopefuls like Carly Fiorina made their pitch to voters too, reminding them that polls are notoriously unreliable in Iowa, where political upsets are commonplace.
The three Democrats and 12 Republicans aiming to be their party's 2016 torchbearer are leaving it all on the field in Iowa, hosting several dozen events as they gear up for Monday's vote.
The state is not large in population and it is relatively homogeneous, but it is immensely consequential for the top finishers who can claim momentum heading into the primary February 9 in New Hampshire.
On the Republican side, it is billionaire Trump at the fore, tearing up the traditional playbook and largely avoiding the retail politics that require candidates to put in days and weeks in Iowa.
But he made the requisite appeal to evangelicals, who comprised 57 percent of caucus voters in 2012 and are expected to play a huge part in the February 1 vote.
Trump posted a short video on Facebook, showing him holding up a Bible given to him by his mother.
"I want to thank the evangelicals. I will never let you down," he said.
Trump displayed his usual confidence at a rally Saturday, declaring, "If we win Iowa, we can run the table!"
But, like other candidates, he moved to soften expectations as the vote grew nearer.
"I don't have to win it," he told CBS's "Face the Nation." "But I think it would be really good to win Iowa. I'd like to win Iowa."
Cruz, meanwhile, is locked in a do-or-die battle with Trump in the state, and is counting on a strong evangelical turnout to help propel him to victory.
"We need godly wisdom back in the White House," supporter Pam Cobb said at a Cruz rally in Ida Grove, in northwest Iowa.
'Right to be angry'
Hovering in third place among Republicans is Rubio, whose star is seen as rising perhaps just at the right time.
"You have a right to be angry," the Florida senator told more than 300 people at a university hall in Ames, Iowa.
"But anger is not a plan," he said in a dig at the bombastic Trump, portraying himself as the most electable Republican.
Clinton took the stage at Iowa State University in Ames with her daughter Chelsea, along with former congresswoman Gabby Giffords, a gun violence victim who has helped raise concerns about the issue.
"Hillary is tough," Giffords said by way of introduction. "In the White House, she will stand up to the gun lobby."
Sanders, for his part, held a raucous rally Saturday night in Iowa City with rockers Vampire Weekend.
"I believe it would be powerful to have a woman president," said Corinne Fonteyne, who is 17 but will be eligible to vote in November. "But Bernie is the way I go."
Clinton has earned a coveted, if not unexpected, endorsement from The New York Times, but she also has had to respond to renewed questions about her handling of classified material as secretary of state.
"It's clear they are grasping at straws," she said of Republicans who have made an issue of her use of a private email server.