Republican heavyweights come out against Trump

Former Republican presidential nominee Mitt Romney on Thursday launched a scathing attack on Donald Trump, the current Republican frontrunner, in an attempt to block his path to the party's presidential nomination.

Trump's nomination as the Republican presidential candidate would "greatly diminish" prospects for a safe and prosperous future for the United States, Romney said in a speech.

Romney said the other Republican candidates would be better alternatives to the billionaire businessman, whom he called "a phony, a fraud."

It is virtually unheard of for a former party standard-bearer to attack a fellow party member and candidate for the presidential nomination. But it has been a political season like none before.

The Republican race, already dominated by insults and name-calling, has seen Trump's once-unlikely candidacy morph into an increasingly strong bid for his party's nomination for the November election.

"The only serious policy proposals that deal with the broad range of national challenges we confront today come from Ted Cruz, Marco Rubio, and John Kasich," Romney said of Trump's rivals. "One of these men should be our nominee."

Romney was relentless in his criticism, saying Trump "is not the temperament of a stable, thoughtful leader. His imagination must not be married to real power."

The attack comes after Trump made serious gains in the Super Tuesday primaries, raising alarm bells within the Republican establishment.

Panicked Republican leaders say they still have options for preventing Trump from winning the nomination, just not many good ones.

Trump is coming under increasing pressure from his party as he fights for the majority of delegates needed to win the nomination.

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

Romney also criticised Clinton, accusing her and her husband, former President Bill Clinton, of personally profiting from their positions of power.

Arizona Sen. John McCain, the losing Republican nominee in 2008, joined the attack on Thursday by issuing a statement endorsing Romney's remarks.

Panicked Republican leaders say they still have options for preventing 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.

Also Thursday, dozens of conservative national security experts warned that Trump is unfit to be commander in chief.

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

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

Many observers believe that the Republican establishment has failed to challenge Trump until it was far too late, and current attempts to undermine his will only shore up his support base, which is already angry at establishment politics.

Agencies contributed to this report