US Democratic presidential candidate Hillary Clinton greets supporters at a campaign rally in Des Moines, Iowa January 31, 2016.[Photo/Agencies]

"We can and we will get back to the founding principles that made America great," Cruz said on Twitter.

A win for Trump could validate a campaign that has alarmed the Republican establishment, dwarfed the efforts of many seasoned politicians and been marked by controversies such as his calls for a temporary ban on Muslims entering the United States and for a wall along the Mexican border.

Iowa has held the first nominating contests, called caucuses, since the early 1970s, giving it extra weight in the US electoral process that can translate into momentum for winning candidates. The caucuses are voter gatherings that take place in 1,100 schools, churches and other public locations across the Midwestern state.

The 2016 election is shaping up to be the year of angry voters as disgruntled Americans worry about issues such as immigration, terrorism, income inequality and healthcare, fueling the campaigns of Trump, Sanders and Cruz.

For the Democrats, Clinton needs a win in Iowa to prevent a potential two-state opening losing streak that would raise fresh questions about a candidate who was considered the clear front-runner just two months ago. Sanders is leading in polls in New Hampshire, the next state to hold a nominating contest.

Clinton began her day at her campaign's south Des Moines field office, where she served roughly 60 volunteers donuts and coffee. An extensive ground operation, much like Obama's as a candidate, has been cited as one of her advantages.

A former US senator and first lady, Clinton often touts her years of experience in politics, and says she will defend much of Obama's legacy. Sanders has attacked from the left and promised to do more than Clinton to help American workers.

Clinton, who lost Iowa in 2008 and went on to lose a protracted primary battle to Obama, told ABC's "Good Morning America" program that it would be different this time, adding, "I think I'm a better candidate."

Sanders said he was pleased with his performance.

"We're very proud of the campaign we ran," he told CNN.