US Republican presidential frontrunner Donald Trump engaged his rivals in a heated exchange on foreign policy as the fourth GOP presidential debate ended on Tuesday night in Milwaukee with no "game-changers," according to pollsters.

The most relieved candidate after the two-hour encounter inside the Milwaukee Theatre, according to Reuters, was former Florida governor Jeb Bush, who failed to make a dent in the three previous debates and had seen a decline in financial donations.

Trump gave Bush the chance to speak strongly when the billionaire businessman asserted that the United States did not need to defend Ukraine against Russian President Vladimir Putin.

"I'm all for protecting Ukraine and working, but we have countries that are surrounding the Ukraine that aren't doing anything. They say 'keep going, keep going, you dummies. Keep going, protect us,' and we have to get smart," Trump said. "We can't continue to be the policeman of the world."

Bush shook his head. "Donald is wrong on this," he said. "Absolutely wrong on this. We're not going to be the world's policeman, but we'd better sure as heck be the world's leader. That's a huge difference."

Bush also pointed out that Trump was wrong in backing Putin's action in Syria, saying Trump's views of Putin and his policies in Syria were "like a board game ... like playing Monopoly or something. That's not how the real world works."

Trump hit back: "Look at mess we have after spending $2 trillion, thousands of lives, wounded warriors all over the place."

The remark brought in Carly Fiorina, who said that she would not negotiate with Putin because "we are speaking to him in a position of weakness brought on by this administration."

Fiorina said she would take steps to build up U.S. military forces around the world.

Marco Rubio then butted in, calling Putin "a gangster" who was "testing the weakness" of President Obama.

"He has seen what the president has done," he said. "The president has no strategy, our allies in the region do not trust us."

Trump also came under fire for his immigration plan, which envisions building a wall on the border with Mexico and round up and deport undocumented immigrants.

Ohio Governor John Kasich and Bush both blasted Trump's plan, which many Republicans fear will alienate Hispanic voters who are vital to winning the election.

"That's the problem with this. We need to win the presidency, and the way you win the presidency is to have practical plans," Bush said.

Pollsters and pundits had different opinions on the candidates following the debate.

Fergus Cullen, a former chairman of the New Hampshire Republican Party, said "Jeb Bush was much improved. At a minimum, this buys him more time to reset and try to advance," according to Reuters.

However, pollster John Zogby had a totally different opinion, telling Newsmax TV that Rubio was the big winner in Tuesday's debate, while his mentor, Bush, was the big loser.

Rubio's win was "followed by a very strong Carly Fiorina, and a stronger than usual John Kasich," Zogby said during post-debate coverage.

However, he said he was "not sure there are game-changers tonight," adding that "it's almost irrelevant to talk about debate performances" when Trump and his close rival Ben Carson "seem to be immune to debate performances."

For his part, political strategist Dick Morris told NewsmaxTV that Carson's dismal performance in Tuesday's debate proved that the retired paediatric neurosurgeon is in "no way" a presidential candidate.

"The big question that hangs over me as I saw this debate is what the hell is holding Ben Carson up?" Morris told host J.D. Hayworth on the "NewsmaxTV Debate Special."

"You look at this guy and there's no way that he's a presidential candidate."

"There isn't the power there," Morris said. "There isn't the strength there. When he talks about national defence, it's like he's painting by numbers.

"He obviously doesn't know what he's talking about with the economy or about national defence — and why should he?" Morris asked. "It's not his field, and I just can't for the life of me understand it."