Republican Donald Trump and Democrat Hillary Clinton have narrow leads over their respective opponents in Iowa, the crucial state which formally kicks off the American presidential elections with the beginning of the primaries tomorrow.

However, the final polls before the crucial Iowa caucus, released by the prestigious Des Moines Register newspaper in association with Bloomberg reveal a different story for the two front runners – Trump and Clinton.

While Trump has overtaken Senator Ted Cruz from Texas in this latest Iowa poll, Clinton has a slim edge over Senator Bernie Sanders.

"Donald Trump has muscled ahead in Iowa, regaining his lead on the brink of the first votes being cast in the 2016 presidential race," the Des Moines Register said.

Trump has support of 28 per cent, while Cruz follows him with 23 per cent. In a previous poll released on January 13, Trump (22 per cent) had trailed Cruz (25 per cent).

"Trump is leading with both the inner core of the caucus universe and the fringe - that's what any candidate would want," said veteran Iowa pollster J Ann Selzer, who conducted the survey.

On the other hand, the poll revealed that Clinton's lead over Sanders is very slim. The former secretary of state is the favourite of 45 per cent of prospective Democratic caucus-goers, while 42 per cent support Sanders. As a result, many see this as a dead heat in the Democratic Party.

"This race is as tight as can be. If Bernie Sanders had momentum headed into the final month, the race now is static and essentially tied," said national political strategist David Axelrod.

The poll conducted by Selzer has long been influential and accurate, Politico reported. Selzer's surveys predicted victories by Barack Obama and Mike Huckabee in 2008, and foresaw Rick Santorum's late surge in 2012, it said.

Meanwhile, the influential New York Times daily has endorsed Clinton and John Kasich as the respective Democratic and Republican presidential candidates.

The decision of the NYT editorial board on Clinton was not surprising, but the top US daily surprised many by rejecting the credentials of top the three Republican aspirants including its city resident Trump who has been leading all major polls ever since he declared his candidacy last summer.

"Hillary Clinton is the right choice for the Democrats to present a vision for America that is radically different from the one that leading Republican candidates offer — a vision in which middle-class Americans have a real shot at prosperity, women's rights are enhanced, undocumented immigrants are given a chance at legitimacy, international alliances are nurtured and the country is kept safe," the daily said.

Rejecting the candidacy of Sanders, NYT said the Vermont senator does not have the breadth of experience or policy ideas that Clinton offers.

"His boldest proposals — to break up the banks and to start all over on health care reform with a Medicare-for-all system — have earned him support among alienated middle-class voters and young people. But his plans for achieving them aren't realistic like Mrs Clinton," it said.

To the surprise of many the top daily rejected the presidential aspirations of the top two Republican candidates Donald Trump and Ted Cruz arguing that they are "equally objectionable" for different reasons.

While Trump has neither experience in nor interest in learning about national security, defence or global trade, the daily said, adding that Cruz's campaign isn't about constitutional principles but about ambition.

"Gov John Kasich of Ohio, though a distinct underdog, is the only plausible choice for Republicans tired of the extremism and inexperience on display in this race. And Mr Kasich is no moderate. As governor, he's gone after public-sector unions, fought to limit abortion rights and opposed same-sex marriage," the NYT editorial said.

Real estate tycoon Trump, who is leading all major opinion polls as the Republican front runner, is also the most unpopular candidate, a Gallup poll has said.

"Trump is the most unpopular candidate of either party when the entire US population is taken into account," Gallup said releasing its own poll in which 60 per cent of Americans view the Republican front runner unfavourably.

It said Trump has a higher unfavourable rating than any nominated candidate from either of the two major parties going back to the 1992 election when it began to track favourability using the current format.

At this point, 33 per cent of Americans view Trump favourably and 60 per cent unfavourably on a two-week average.

Clinton currently has a 52 per cent unfavourable rating among all Americans, while Jeb Bush is at 45 per cent, Chris Christie 38 per cent, Ted Cruz 37 per cent, Marco Rubio 33 per cent, Sanders 31 per cent and Ben Carson 30 per cent.

"Trump's 60 per cent is clearly well above all of these. Putting his favourable and unfavourable ratings together yields a net favourable of -27 for Trump, far above the -10 for Clinton and for Bush, the next lowest among the major candidates," Gallup said.

Compared when George W Bush was running for re-election in 2004, his highest unfavourable was 47 per cent, while John Kerry's highest was 45 per cent. Obama's unfavourable rating in 2008 was 37 per cent and in 2012 it jumped to 48 per cent.

However, independent Ross Perot's unfavourable rating reached above 60 per cent in both the 1992 and 1996 elections, the poll said.