The central state of Iowa opens voting Monday in the first U.S. contest to pick Republican and Democratic presidential nominees, leading to November's national election.
The two poll-leading Republican presidential contenders, billionaire real estate mogul Donald Trump, conservative Texas Senator Ted Cruz, and several other candidates headed to more rallies as voting neared.
Election caucuses were set for 1,681 schools, firehouses and community centers throughout the state.
The top Democratic contenders, former Secretary of State Hillary Clinton and Vermont Senator Bernie Sanders, are locked in a tight contest for their party's nod in the first of a long series of state-by-state contests.
A confident Clinton, is clinging to a narrow edge in Iowa over Sanders, the self-described independent Democrat socialist.
Based on the state votes, each party will formally pick its nominees at national conventions in July, with the winners squaring off in the November election.
Iowa, geographically in the central part of the United States, is not a large state and at most a few hundred thousand voters are expected to take part in the caucuses.
But with its status as the first to vote in the lengthy presidential campaign, the Iowa results will give U.S. political pundits, a vast array of commentators on television and radio shows, newspapers, magazines and Internet blogs, a chance to offer a myriad of opinions on the outcome late Monday and in the coming week, before the next state, New Hampshire in the northeastern United States, votes February 9.
WATCH: VOA Snapshot of Iowa Race - Voters
Opinion polls in Iowa showed a tantalizingly close race for both the Republican and Democratic Party nominations, with non-establishment candidates continuing to show resiliency.
On the Republican side, Trump maintains a slight lead over Cruz, who appears to have made several missteps in the lead-up to the Iowa voting, with Clinton and Sanders in a statistical dead heat.
Candidates in both parties made a final sprint through Iowa Sunday, holding events in every corner of the state, stepping up attacks on rivals and making last-minute appeals to key voter groups.
Sunday morning TV network news shows also were a key platform. On ABC’s This Week, Trump tore into his main challenger Cruz, calling him a “total liar” and a “nasty guy.”
Trump also continued to raise questions about whether Cruz is eligible to run for president, because he was born in Canada to a Cuban father and an American mother.
Cruz shot back, challenging Trump’s conservative credentials on issues such as health care, and accusing Trump of running an immature campaign.
Cruz’s chances to win the Iowa caucuses are seen as slightly diminished after he suffered a shaky debate performance last week. His campaign also was on the defensive after sending out a campaign flier that appeared to threaten voters if they did not show up to vote.
Trump and Cruz are both trying to portray themselves as political outsiders, and are competing to attract evangelical Christians who comprise a large percentage of Iowa Republicans.
WATCH: VOA Snapshot of Iowa Race - Republicans
Sanders attacks Clinton on emails
Meanwhile, Sanders showed an increased willingness to raise questions about Clinton’s use of a personal email server during her time as secretary of state, from 2009 to 2013.
After months of refusing to make the issue a point of attack on the campaign trail, Sanders told CNN that the controversy is a “very serious issue.”
The U.S. State Department, which has been releasing Clinton’s emails in batches, on Friday said it would not publish 22 Clinton emails because they contained top secret classified information. Clinton's critics say her use of the private email server put U.S. national security at risk, and may even have violated some laws.
Clinton has said she did nothing wrong and that the emails were not marked classified at the time she received or sent them. On Sunday, Clinton repeated her stance the emails should be made public, and she slammed her Republican critics for focusing on the issue.
WATCH: VOA Snapshot of Iowa Race - Democrats
Another debate planned
The Sanders and Clinton campaigns on Sunday also reached an agreement with the Democratic National Committee (DNC) to hold additional debates, something many in the party had wanted.
The DNC says the next officially sanctioned debate will be held Thursday in New Hampshire, where Sanders has a substantial double-digit lead over Clinton.
Iowa storm coming?
But for now, all eyes are on Iowa, where a major snowstorm may hit about the time residents gather at caucus centers to vote late Monday.
The storm could impact voter turnout, providing an advantage for better organized campaigns or those with more passionate supporters.
“I think the conventional view here is that if there’s high turnout, that is great news for Bernie Sanders,” says Kathie Obradovich, a political columnist for The Des Moines Register.
“When it’s a small turnout event, anything can happen. So, bad weather would add to the uncertainty of what happens on caucus night,” she said.