Populist Trump, who has surprised all political pundits in the US with a sustained campaign he has run so far, has been leading all opinion polls since he announced his candidature last summer.
He was topping the Iowa polls with 37 per cent, according to CNN/ORC poll. His nearest rival 49-year-old Ted Cruz, from Texas and a Tea Party favourite, is trailing behind at 26 per cent.
With all eyes on Iowa Caucus on February 1, which would formally kick off the 2016 presidential elections, followed by New Hampshire primary a week later, candidates from both the Democratic and Republican parties have virtually stormed the two crucial states, fanning to small villages and cities in their last-minute efforts to convince voters to support them.
Trump, 69, is leading considerably in both the states, but this is not the case with 68-year-old Clinton, who is eyeing to create history in the November 2016 presidential elections by becoming the first woman to be elected as the US president.
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She had a commanding lead over her nearest Democratic rival Sanders in Iowa until December. As per the CNN/ORC poll, Clinton led Sanders by 18 points in December.
But as per the latest opinion poll released by the same agency, Sanders now leads Clinton by eight points.
In her second presidential bid, a defeat in Iowa could be considered a major setback for the Clinton campaign, as this is traditionally expected to set the pace and tone of the rest of the presidential season.
Nationally, Clinton still has a considerable lead over Sanders, 74.
As per RealClearPolitics, which aggregates all the national polls, Clinton has a 13-point lead over the Senator from Vermont.