Donald Trump’s maniacal bid to become the next president of the United States keeps hitting new lows.

After a tumultuous weekend of campaigning during which Mr Trump refused to take any responsibility for violence at his rallies, the New York billionaire declared that he was considering stumping up legal assistance for a supporter who last week assaulted and threatened to kill a black protester.

Trump supporter John McGraw has been charged with assaulting Rakeem Jones during a rally in North Carolina earlier this month.

After the incident, Mr McGraw told a television journalist that “the next time we see him, we might have to kill him”.

In a remarkable development on Monday (AEST), Mr Trump told NBC Television’s Meet The Press that he had instructed his legal team to look into ways to help Mr McGraw with his legal fees.

At a rally in Iowa last month, Mr Trump promised supporters he would pay their legal fees if they were arrested for “knocking the crap out” of protesters.

The Republican frontrunner’s inflammatory rhetoric has horrified the organisational wing of his party who are worried that his eventual nomination could give Democrat favourite Hillary Clinton an easy ride to the White House.

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Mr Trump’s three rivals in the Republican primary race hit out at the billionaire’s failure to moderate his language at campaign events.

“We are now a nation where people hate each other,” senator Marco Rubio said.

“We are now a nation where we are no longer capable of debating serious public policy without immediately concluding that the person who disagrees with you is evil. This can’t continue.”

Polls indicate Trump is losing momentum

This week’s primaries across five states loom as critical tests for Mr Trump’s quest for the Republican nomination.

Ted Cruz is closing on Trump in Illinois and Missouri, polls show. Photo: Getty

According to most polls, he is set to clean up in Florida and North Carolina, but recent surveys of voters show that support for him is tapering in Ohio, Illinois and Missouri.

In fact, separate polls commissioned by FoxNews and NBC at the weekend show Governor John Kasich drawing ahead of Mr Trump in Ohio.

Mr Trump is also under pressure in Illinois and Missouri where polls show big late swings to Texas senator Ted Cruz.

A CBS poll taken before protesters forced Mr Trump to cancel a rally in Chicago last Friday indicates the contest will be a nail-biter.

In a sign that Mr Trump’s campaign might be unravelling in Illinois, it was announced on Monday that state campaign director Kent Gray had been sacked.

Results of each of this week’s round of primaries will be known early on Wednesday afternoon (Australian time).

National polls show Trump cannot win the Presidency

The Republican Party’s panic over Mr Trump getting the nomination is grounded in Realpolitik.

Every national poll taken since the beginning of February shows support for Mr Trump collapsing in projected races against Democratic candidates Hillary Clinton and Bernie Sanders.

According to an NBC poll taken last week, Mr Trump would lose to Mrs Clinton by 51 per cent to 38 per cent.

The same poll found Mr Sanders would win by 55 per cent to 37 per cent.

However, presidential contests between Mrs Clinton and any of Mr Trump’s Republican rivals would be very close, the poll found.

Sanders is surging in some states. Photo: EPA

A key swing factor working against Mr Trump in recent weeks is the apparent loss of support among Republican women.

The Michigan primary held last week showed that only 28 per cent of Republican women voting for Mr Trump, compared to 45 per cent of men.

Late surge for Bernie in Illinois and Ohio

Mrs Clinton seems headed for massive victories in Florida and North Carolina this week, but polls suggest that she is vulnerable to late surges in support of Mr Sanders in other states.

Mrs Clinton might even lose Illinois – the state in which she was born and raised.

A CBS poll conducted last week has Mr Sanders drawing ahead of Mrs Clinton by two per cent.

Mrs Clinton is holding on to leads in Ohio and Missouri, but her margins in both states have narrowed dramatically since the end of February.