US Republican and Democratic candidates made last-gasp appeals to Iowa voters just hours before the first nominating contest of the 2016 campaign, with their White House aspirations on the line.

By late yesterday, presidential hopefuls will learn whether the time, effort and money they poured into their campaigns have been rewarded with the kind of strong showing here that would lend huge momentum to their cause when the race shifts toward the next vote, in New Hampshire.

Republican Donald Trump and Democrat Hillary Clinton both were leading in the polls, but not by enough to assure victory for either candidate in yesterday's caucuses.

Will an establishment Republican -Marco Rubio, perhaps - outperform the polls and make it a three-way race with Trump and conservative Ted Cruz? And will self-declared Democratic Socialist Bernie Sanders convert the excitement about his political revolution into enough votes to upset Clinton's second shot at history?

Iowa - and early momentum in the presidential race - is up for grabs. Most of the 12 Republicans and three Democrats in the race were holding several weekend events to press the flesh and win over voters.

By evening, several leading candidates were rallying their faithful, pleading with them to defy snow or freezing rain and bring friends and neighbors to caucus. Clinton, seeking to stave off a surge by Sanders, was leaving nothing to chance, stumping in the heartland this weekend and warning that the policy proposals of her main rival were pie-in-the-sky idealism.

Sanders, a senator from Vermont, declared his race with Clinton a "tossup." A Bloomberg/Des Moines Register poll published late Saturday put Clinton just ahead of Sanders in Iowa, 45 to 42 percent. The same poll gave Trump a bigger lead over chief Republican rival Cruz, a senatorfromTexas.