The billionaire businessman has harnessed working-class anxieties about immigrants and trade to supercharge his bid to become the Republican Party's candidate.
U.S. Republican presidential front-runner Donald Trump called for raising tariffs on foreign-made goods on Thursday, in a further break from his party's long-standing free-trade philosophy ahead of nominating contests in big industrial states hit hard by globalization.
The billionaire businessman has harnessed working-class anxieties about immigrants and trade to supercharge his bid to become the Republican Party's candidate in the November election. His White House bid has driven a wedge between the party's moneyed donors and grassroots voters, alarming party leaders who see him as a dangerous demagogue.
His raucous rallies occasionally descend into violence, including an attack on Wednesday night on a black protester in North Carolina.
Trump, Ted Cruz, Marco Rubio and John Kasich will share the stage at a CNN-hosted debate at the University of Miami, which begins at 8:30 p.m. (0130 GMT on Friday). The event follows a debate last week that degenerated into name-calling and sexual innuendo.
Cruz, a first-term U.S. senator from Texas, is known for antagonizing fellow Republicans in Washington. On Thursday, he snared his first endorsement from a Senate colleague.
"Ted doesn't believe you have to settle," U.S. Senator Mike Lee of Utah told a Miami news conference.
Cruz, 45, aims to push Rubio, a first-term U.S. senator from Florida, and Ohio Governor Kasich off the campaign trail in order to position himself as the best Trump alternative.
"We think it's a two-man race," said Cruz backer Saul Anuzis, a former chairman of the Michigan Republican Party.
For his part, Trump directed his ire overseas, saying the United States should raise tariffs when countries in Europe or Asia lower their interest rates.
"We just sit back and do nothing," Trump, 69, said on CNBC television. "That's getting to be very dangerous as far as I'm concerned."
Trump's trade-sceptical message could help him next Tuesday in primaries in Ohio, Illinois and North Carolina, which have lost hundreds of thousands of manufacturing jobs to foreign competition. Florida, Missouri and the Northern Mariana Islands, a U.S. commonwealth in the Pacific, also hold nominating contests that day.
Though the brash New Yorker has made economic issues a central campaign theme, he has steered clear of specifics.
He has promised to deport 11 million illegal immigrants, temporarily bar Muslims from entering the country and renegotiate trade deals with countries like China to protect American workers.
Critics have dismissed those plans as unworkable at best, and Trump's Republican presidential rivals have pointed out that his resorts have brought in workers from abroad rather than hiring Americans.
Our Principles PAC, an anti-Trump group, said on Thursday it would spend $1 million on ads in Ohio - where Republicans will vote in a crucial presidential primary next week - that hit him for having shirts and ties made in Asia.
Those attacks have yet to slow him down. After nominating contests in 25 states and Puerto Rico, Trump has amassed 458 delegates, according to the Associated Press, ahead of the 359 won by Cruz and the 151 won by Rubio. Kasich trails with 54 delegates.
Trump needs 1,237 delegates to clinch the nomination. Mainstream Republicans say their best bet at this point is to stop him from reaching that number, which could enable a compromise candidate to emerge at the party's July nominating convention in Cleveland.
Rubio, 44, and Kasich, 63, face make-or-break contests in their home states next Tuesday, and both will face intense pressure to drop out if they lose. Polls show Kasich running competitively with Trump in Ohio, but Rubio lags far behind in Florida.
In Wednesday night's violence, sheriff's deputies arrested a 78-year-old white man on charges he punched the 26-year-old black protester in the face at the rally in Fayetteville, North Carolina, the local sheriff's office said on Thursday. The young man was attacked as deputies escorted him away from the event.
"The next time we see him, we might have to kill him," the suspect, John McGraw, told the national television show "Inside Edition." The day before that incident, Trump's campaign manager, Corey Lewandowski, grabbed a reporter for the conservative Breitbart News at an event, hard enough to leave bruises on her arm, according to the Washington Post. Trump's campaign denied the incident happened, and Lewandowski called the reporter, Michelle Fields, an "attention seeker."
Trump has occasionally encouraged mayhem. "I'd like to punch him in the face," he said of a protester at a February rally in Las Vegas. Democratic presidential front-runner Hillary Clinton, 68, said she was "appalled" by the violence.
"This is deeply distressing, and I think as the campaign goes further, more and more Americans are going to be really disturbed by the kind of campaign he's running," she said on MSNBC. Clinton is vying with Senator Bernie of Sanders, 74, for her party's nomination.
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