Republican old guard Romney labels Trump as 'a fraud'

Republican US presidential candidate Donald Trump speaks about the results of Super Tuesday primary and caucus voting during a news conference in Palm Beach, Florida March 1, 2016. [Photo/Agencies]

Ahead of the speech, much of which had been previewed in excerpts, Trump dismissed Romney in television interviews and posts on Twitter, calling him "a failed candidate" who had "begged" him for an endorsement in 2012.

"Mitt Romney is a stiff," Trump told NBC's "Today" program.

Romney's strategy risks backfiring by further energizing Trump's supporters, who are angry with a party they see as not defending their interests.

"If you're Trump, this is like getting the good kind of Kryptonite," Republican strategist Doug Heye said.

Romney's speech came hours before Trump and rivals Marco Rubio, Ted Cruz and John Kasich share a debate stage in Detroit.

It risks locking Trump's opponents out of news coverage until the 9 p.m. EST (0200 GMT Friday) debate hosted by Fox News begins. The debate will be the candidates' first face-to-face gathering since Super Tuesday nominating contests this week gave extra momentum to Trump but did not knock out his rivals.