Clinton barks, Trump calls Cruz 'unstable', Bush comes to help Jeb!

Washington: Hillary Clinton barked like a dog, Donald Trump called Ted Cruz "unstable," and Jeb Bush brought his big brother George to vouch for him as the US presidential race heated up ahead of the battle of South Carolina.

Accusing Republicans of saying things that are not true, the Democratic frontrunner Hillary Clinton on Monday told a colourful story in Reno, Nevada, about a radio ad that she said ran in rural Arkansas while her husband, Bill Clinton, was running for office.

In the ad, as Clinton remembered, a trained dog barked if something was untrue. "I want to figure out how we can do that with Republicans," she said as quoted by CNN.

"We need to get that dog and follow them around and every time they say these things like, 'Oh, the Great Recession was caused by too much regulation,' arh, arh, arh, arh," Clinton said, letting out a barking noise that caused the audience to laugh and some people to mimic her.

"I think," Clinton concluded, "we could cut right through a lot of their claims."

Clinton's comments came ahead of Saturday's Democratic caucuses in Nevada and the Republican primary in South Carolina. Republicans hold their caucus in Nevada on February 23, and the Democrats their primary in South Carolina on February 27.

On the Republican side, frontrunner Trump called rival Cruz "unstable", threatening a lawsuit "if he doesn't take down his false ads and retract his lies" and urged the Republican National Committee (RNC) to "intervene".

He also threatened to renege on his pledge to not run as independent, saying that the Republican establishment was not holding up its end of a deal.

"I signed a pledge and as far as I'm concerned they're in default of their pledge," Trump said in Charleston, South Carolina.

"He's a very bad guy. He's a very nasty guy. He's got no endorsements from any senators who he works with. He'll never be able to unify anything," he said of Cruz later

Trump acknowledged to CNN that his brawl with Cruz might harm the eventual Republican nominee, whoever it is, "but I think we'll get over it".

The Cruz campaign responded by saying Trump is lashing out "with name-calling and falsehoods".

Meanwhile, former president George W. Bush, who left office in 2009, came to the aid of his brother taking a clear swipe at Donald Trump during a campaign rally in North Charleston, South Carolina.

"We do not need someone in the Oval Office who mirrors and inflames our anger and frustration," he said asking voters to choose a candidate who is "measured and thoughtful" over the loudest and angriest man in the room.

"We need someone that can fix the problems that cause our anger and frustration, and that's Jeb Bush," he said describing the 2016 campaign a "serious election for a serious job".

The former president pointed to the abundant "name-calling" in the 2016 election -- an apparent reference to the punches thrown between Jeb Bush and Trump in recent months -- and joked that according to their father, George H.W. Bush, "labels are for soup cans".

RNC also pushed back at Trump's claim that the loud boos that greeted him when he criticised George Bush during the presidential debate on Saturday came from an audience stacked with big money donors.

The New York Times cited a senior official with the South Carolina Republican Party as estimating that there were no more than 300 donors among the 1600 people in the audience.