Former US presidential candidate Mitt Romney condemns 'phoney' Donald Trump

Donald Trump has come under a brutal attack from former US presidential nominee Mitt Romney, who urged fellow Republicans to shun the property tycoon for the good of the country and party.

Mr Romney, who was soundly defeated by President Barack Obama in his 2012 re-election, joins a growing chorus of anxious Republican leaders that many Trump supporters see as establishment figures.

"Here's what I know: Donald Trump is a phoney, a fraud," Mr Romney says in a speech set for delivery later.

Mr Romney will say a Trump nomination at the party's convention in July would enable Democrat Hillary Clinton to win the presidency.

Mr Romney will also say Mr Trump "has neither the temperament nor the judgement to be president".

Mr Trump disparaged Mr Romney in a series of tweets, including "I am not a Mitt Romney, who doesn't know how to win."

Panicked Republican leaders say they still have options for preventing Mr Trump from winning the nomination, just not many good ones. They include a contested convention and even the long-shot prospect of a third party option.

Meanwhile, dozens of conservative national security experts have warned that Mr Trump is unfit to be commander in chief.

Former Homeland Security Secretary Michael Chertoff and more than 70 others called Mr Trump's "embrace of the expansive use of torture" inexcusable.

They also object to what they say is Mr Trump's "hateful, anti-Muslim rhetoric" and his advocacy for waging trade wars.

Mr Trump has 316 delegates so far, Texas Senator Ted Cruz 226 and Florida Senator Marco Rubio, 106. It takes 1,237 delegates to win the party nomination.

Mr Trump is already setting his sights on the general election.

His campaign reached out to House Speaker Paul Ryan's office to arrange a conversation between the two men and urged Republican leaders to view his candidacy as a chance to expand the party.

"Why can't the leaders of the Republican Party see that I am bringing in new voters by the millions - we are creating a larger, stronger party", Mr Trump tweeted.

Indeed, there was a surge in turnout in Super Tuesday's primaries.

While that could be a welcome sign for a party that has struggled to attract new voters in recent presidential elections, Republican leaders face the reality that some of those voters were registering their opposition to the party establishment.

Party strategists cast March 15 as the last opportunity to stop Mr Trump through the normal path of winning states and collecting delegates.

A win for Mr Rubio in his home state of Florida would raise questions about Mr Trump's strength, as could a win for John Kasich, Ohio's governor, on his home turf.

The candidates have a high-profile opportunity to make their case to voters in Thursday night's prime-time debate.

Retired neurosurgeon Ben Carson all but ended his bid on Wednesday, saying he would skip the debate and declaring he did "not see a political path forward".

On the Democratic side, former Secretary of State Hillary Clinton was drawing broad support from voters and her party's leaders.

Rival Bernie Sanders vowed to keep up the fight, though his path to the nomination has narrowed.

So far, Mrs Clinton has at least 1,005 delegates, and Mr Sanders 373. It takes 2,383 Democratic delegates to win.