By John Whitesides, Ginger Gibson and Steve Holland
DES MOINES, Iowa (Reuters) - Donald Trump's ground-breaking White House campaign faces its first test at the hands of voters on Monday when Iowa begins the nationwide process of choosing a new U.S. president, with polls showing him in a tight battle with Ted Cruz to be the Republican nominee.
On the Democratic side, front-runner Hillary Clinton is hoping to fend off a stiff challenge in Iowa from insurgent Bernie Sanders in the first of the state-by-state battles to pick party candidates for the Nov. 8 election to succeed President Barack Obama.
Late opinion polls showed Trump, a blunt-spoken billionaire businessman who has never held public office, with a small lead over Cruz, a conservative U.S. senator from Texas. Clinton had a slight edge on Sanders, a U.S. senator from Vermont.
But there is a large bloc of undecided voters in both parties in Iowa and no certainty on who will turn up at the caucuses on a wintry evening, given that many supporters of Trump and Sanders are new to the process and disenchanted with traditional politics.
A Des Moines Register/Bloomberg Iowa poll on Saturday showed three in 10 likely Democratic caucus-goers and 45 percent of likely Republican caucus-goers were still uncertain.
Iowans will begin choosing candidates at 7 p.m. CST (0100 GMT on Tuesday), with results expected within a few hours.
A win for Trump could validate an aggressive campaign that has alarmed the Republican establishment, dwarfed the efforts of many seasoned politicians and has 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.
A loss would dent the campaign persona of "winner" that the real estate tycoon and former reality TV star has carefully cultivated, and create immense pressure for a better performance in the next nominating contests - in New Hampshire and South Carolina - later this month.
'TAKING OUR COUNTRY BACK'
Trump started the day with a rally in Waterloo, Iowa, urging people to turn out. He told the crowd that while he is leading all the polls, none mattered now if they did not attend caucuses.
"Tonight is so important: this is the beginning of taking our country back," said Trump.
At a later rally in Cedar Rapids, he said his security had warned him there might be protesters with tomatoes, and responded in typically pugnacious fashion.
“If you see someone in the crowd getting ready to throw a tomato, knock the crap out of them,” he told the crowd.
The Iowa caucuses kick off a primary process that leads to the parties' formal presidential nominations this summer. The voter gatherings are a long and sometimes arcane ritual, taking place in 1,100 schools, churches and other public locations across the Midwestern state.
With months of heavy exposure to White House hopefuls, many Iowans are slow to decide who to back.
"I'm still checking them out. The field is large and it requires some thought," said Paul Albritton of Carlisle, Iowa, a training coordinator at Iowa State University, as he waited to see U.S. Senator Marco Rubio of Florida last week. "I'm thinking about who can win in November."
For the winners in Iowa, the prize will be valuable momentum that could stretch for months, while many of the losers on the Republican side could quickly begin dropping by the wayside.
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 and self-declared democratic socialist Sanders.
On the Republican side, opinion polls show foreign policy hawk Rubio might win third place in Iowa and stake a claim as the best hope for the party's mainstream.
Jeb Bush, a former Florida governor once thought to be the strongest Republican candidate but watched his poll numbers tumble, held a final Iowa event in Des Moines.
"We have candidates running where it’s all about them, it’s about their ambition," Bush said. "Whenever the going gets tough they’ll, cut and run, because God forbid they actually propose something that currently is not popular."
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.
She began her day at her campaign’s south Des Moines field office, where she brought roughly 60 volunteers donuts and coffee, posed for pictures and then headed to a downtown coffee shop. Building an extensive ground operation has been credited with one of the strongest advantages she has going into the caucuses.
Clinton, a former secretary of state and U.S. senator, 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, but told ABC's "Good Morning America" program that it would be different this time, adding, "I think I'm a better candidate."
(Roporting by Steve Holland, John Whitesides, Amanda Becker and Ginger Gibson in Iowa; Additional reporting by Doina Chiacu and Susan Heavey in Washington; Writing by Alistair Bell; Editing by Caren Bohan, Peter Cooney and Frances Kerry)