Former United States secretary of state Hillary Clinton and real estate tycoon Donald Trump are leading their respective Democratic and Republican parties' presidential races but are locked in a dead heat with their rivals in the key Iowa Caucus, an opinion poll said today.

In Iowa, which would kick off the primary presidential race some two weeks from now, Clinton and her Democratic rival Bernie Sanders are tied at 45 per cent each, as per a CNN/ORG poll.

This is not considered good news for the former secretary of state given her early start to the campaign, a well-oiled campaign machinery and her national recognition.

Nationally Clinton at 54 per cent leads Sanders at 36 per cent by 18 per cent, CNN said.   In a previous poll about a month ago, Clinton (56 per cent) had an advantage of a 25 per cent over Sanders (31 per cent), it said.

A similar story repeats for the Republican party ahead of the Iowa Caucus where Ted Cruz, the Senator from Texas (27 per cent) is in a dead heat with Trump (25 per cent).

Only two other candidates -- Marco Rubio at 14 per cent and Ben Carson at 10 per cent get more than 10 per cent of the votes, while rest of the other candidate are less than five per cent.

However, Trump at 34 per cent leads Cruz at 20 per cent by 14 per cent nationally.

Again only Rubio (12 per cent) and Carson (10 per cent) get more than 10 per cent, while rest of the Republican aspirants do not cross four per cent.