WASHINGTON - Days after Democratic front runner Bernie Sanders' decisive victory in the Nevada caucuses, his rivals called his plans unrealistic and cast doubt that he could triumph over President Donald Trump at Tuesday (Feb 25) night's televised debate.

Criticism of the firebrand Vermont senator from Democrats had been muted until recent weeks, when he picked up steam in national polls and racked up consecutive wins - or a very close second - in the three early races of Iowa, New Hampshire and Nevada.

That was over by Tuesday's debate in South Carolina, where the Democratic primary will be held on Saturday, as they sought to slow his momentum.

Former South Bend mayor Pete Buttigieg, former vice-president Joe Biden and billionaire Michael Bloomberg warned that nominating Mr Sanders would give Mr Trump a second term, lose the Democrats' control of the House and keep the currently Republican Senate out of their reach.

Mr Buttigieg said: "It adds up to four more years of Donald Trump, (current House minority leader and Republican) Kevin McCarthy as speaker of the House, and the inability to get the Senate into Democratic hands."

He added: "If you want to keep the House in Democratic hands, you might want to check with the people who actually turned the House blue (in the 2018 midterm elections) - 40 Democrats who are not running on your platform. They are running away from your platform as fast as they possibly can."

Mr Bloomberg, a former Republican turned Democratic bankroller, said: "Bernie will lose to Donald Trump. And Donald Trump, and the House, and the Senate and some of the state houses will all go red and then between gerrymandering and appointing judges for the next 20 or 30 years, we're going to live with this catastrophe."

Massachusetts Senator Elizabeth Warren said Mr Sanders had not been effective in carrying out his plans, while Mr Biden also went after Mr Sanders for his record on gun violence prevention.

But whether any criticism can shake Mr Sanders from the head of the pack is questionable.

A Reuters-Ipsos poll conducted in the past week (Feb 19-25) and released on Tuesday shows him widening his lead over his other Democratic rivals.

Around 26 per cent of registered Democrats and independent voters surveyed said Mr Sanders was their first choice, with Mr Bloomberg and Mr Biden coming a joint second at 15 per cent each.

Mr Sanders also surpassed Mr Biden in support among African-Americans - 26 per cent to 23 per cent.

This could spell trouble for Mr Biden, a flagging former front runner who is counting on a solid win in South Carolina - where around 40 per cent of Democratic voters are African-American - to stay in the race.

The window is also closing for the other candidates to dethrone Mr Sanders.

If he emerges from the Super Tuesday races on March 3 with an overwhelming lead, he will likely become the Democratic nominee, given the sheer number of pledged delegates - more than a third of all available - that will be allotted through those polls.

Mr Bloomberg, the New York City mayor from 2002 to 2013, was the next most-targeted candidate in Tuesday's debate, coming under sustained fire for his stop-and-frisk anti-crime policy which disproportionately affected minorities, and his past sexist comments.

Ms Warren zeroed in on the billionaire's history of financially backing Republicans in past elections, including her opponent in a 2012 race.

"I don't care how much money Mayor Bloomberg has. The core of the Democratic Party will never trust him," she said.

"He is the riskiest candidate standing on this stage."

In a particularly charged moment, Ms Warren also brought up a 1997 lawsuit against Mr Bloomberg, in which he was alleged to have told a pregnant employee to "kill it" - the baby she was expecting.

He has previously denied making the comment, and again denied doing so on Tuesday. The case was settled with no admission of guilt.

The exchange came on the back of Ms Warren's calling out of Mr Bloomberg at the previous week's Nevada debate for comments in which he called women "fat broads" and "horse-faced lesbians".

Mr Bloomberg, who had previously been rising in the polls, took a hit after his lacklustre debate performance and slipped 2 percentage points in the Reuters-Ipsos poll.

The bigger danger, said Minnesota Senator Amy Klobuchar, was that the Democratic party could be weakened and divided by the nomination infighting, undermining them in the general election.

"If we spend the next four months tearing our party apart, we'll spend the next four years watching Donald Trump tear our country apart," she said.