The remaining four Republican candidates vying to become their party's nominee to run for US president opened the 12th debate of the campaign Thursday, answering questions on trade, education and social benefits for the elderly.

The candidates gathered for the debate in Miami for the last debate before crucial make-or-break primaries in five states on Tuesday.

The candidates disagreed on trade policy, with front-runner Donald Trump complaining about existing trade deals.

"Jobs in this country are disappearing, especially the good jobs," Trump said. "Trade deals are absolutely killing our country."

He criticized Chinese manufacturers who "dump everything they have over here" while not allowing US businesses to set up in China without having to pay a tremendous tax.

Ted Cruz, a US senator from Texas, challenged Trump on his threat to raise tariffs on goods from countries that do not "behave," saying that would hurt US consumers and result in countries the US trades with putting tariffs on US goods.

He said the US has to get beyond the rhetoric of "China bad" and get to solutions.

Much is at stake in the polls on Tuesday, one of the biggest days in the state-by-state voting process thus far. More than 350 delegates will be up for grabs.

Trump will be looking to solidify his status as the front-runner, while Cruz, who is running second, will try to convince Republicans to coalesce around him.

US Senator Marco Rubio of Florida and Ohio Governor John Kasich are the other two remaining candidates. Both Rubio and Kasich are betting on victory in their home states on Tuesday. A Fox News poll released Wednesday showed Kasich leading Trump in Ohio, but Trump topping Rubio in Florida.

The other states where voting will take place on Tuesday are Missouri, Illinois and North Carolina.

The candidates also clashed on their education policies and their plans to revise the social security system that supports people after retirement.

Prior to Thursday's debate Trump pledged a "softer" tone this time around.

Calm prevailed during the first hour of the debate and Trump acknowledged as much from the podium.

"We are all in this together. We are going to come up with the answers to things, and so far I cannot believe how civil it has been up here."

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