Donald Trump in Super Tuesday victory speech
Donald Trump in Super Tuesday victory speechReuters

Republican presidential candidate Donald Trump on Wednesday urged mainstream Republicans to unify behind his candidacy, saying that if Republicans unite around him, he would be “unbeatable”.

The often-brash Trump softened his tone, at least temporarily, hours after securing three more primary victories, praising House Speaker Paul Ryan as a man he respects and encouraging Mitt Romney to promote party harmony, The Associated Press (AP) reported.

"If I win [Florida and Ohio], I think it's over," he was quoted as having told CNN'.

"Instead of fighting it, they should embrace it," he added in another interview on Fox News Channel. "If we embrace what's happening and if everybody came together ...nobody could beat the Republican Party."

AP noted, however, that amid growing resistance to Trump's campaign, a number of former GOP presidential hopefuls have re-emerged in support of his current rivals.

Former Florida Governor Jeb Bush planned to confer with all the candidates save Trump, ahead of Thursday's GOP debate, according to the news agency.

Bush was meeting privately with Florida Sen. Marco Rubio on Wednesday, and planned to meet with Texas Sen. Ted Cruz and Ohio Gov. John Kasich on Thursday, said Kristy Campbell, Bush's former campaign spokeswoman. She provided no details.

Bush and Trump engaged in heated confrontations throughout Bush's campaign, repeatedly referring to each other as a "loser."

Bush dropped out of the race in late February after failing to gain momentum in the primaries.

Meanwhile, Cruz intensified his own bid to blunt Trump's momentum with an endorsement from a former rival, retired business executive Carly Fiorina.

Acknowledging many mainstream Republicans fear Cruz is too conservative, Fiorina told a Miami crowd that she's "horrified" by Trump.

"The truth is that Donald Trump and Hillary Clinton are two sides of the same coin," she charged, according to AP. "It is time now to unite behind the one man who can beat Donald Trump, who can beat Hillary Clinton."