Fourth Republican Presidential Debate: 8 Moments That Mattered

— -- The eight presidential contenders left standing on the main stage faced off over fiscal policy tonight in Milwaukee, Wisconsin -- battling about everything from trade deals to taxes.

The fourth Republican debate, hosted by Fox Business and the Wall Street Journal, drew contrasts among the GOP candidates over spending and economic philosophy.

Here are the eight biggest moments from tonight’s debate:

1. Jeb Bush and John Kasich go after Donald Trump on immigration plan

Trump wants to deport undocumented immigrants -- but Kasich and Bush tried to throw cold water on that plan.

“We are a country of laws,” Trump said. “We have no choice if we're going to run our country properly.”

But Kasich and Bush hit back. “Think about the children,” Kasich said. “So, you know the answer really is? If they have been law-abiding, they pay a penalty. They get to stay.”

“We all know you can't pick them up and ship them across, back across the border. It is a silly argument. It is not an adult argument. Makes no sense,” Kasich said.

Bush also distanced himself from Trump’s idea, calling Trump’s plan “just not possible.” “It would tear communities apart. And it would send a signal that we're not the kind of country that know America is,” he said.

“And even having this conversation sends a powerful signal,” he said, warning of the importance of the general election. “They're doing high-fives in the Clinton campaign right now when they hear this."

2. Donald Trump plays moderator from his podium

Trump may have been in the middle of the debate stage, but that didn’t stop him from playing moderator.

After a back and forth between Trump and Kasich, Bush tried to get a word in asking the moderators, “What happened to my time?” When Kasich didn't stop talking, Trump chimed in:

“You should let Jeb speak,” Trump said, even adding. “No, it’s unfair.”

Kasich continued before the moderators went to the former Florida governor.

“Thank you, Donald for allowing me to speak at the debate,” Bush said, laying on the sarcasm. “Really nice of you. I appreciate that. What a generous man you are.”

And he didn’t stop there. Later in the debate, in a face off between Rand Paul and Carly Fiorina it was Trump who attempted to break in, quipping: “Why does she keep interrupting everybody?”

The comment was greeted by boos.

3. Ted Cruz’s 'oops' moment

It was almost like Ted Cruz didn’t learn anything from Texas politicians. It’s impossible to forget Rick Perry’s “oops” moment from 2011 when he wasn’t able to list the third agency he would eliminate during a Republican debate in Michigan.

It was much less of an oops moment during tonight’s debate, but Cruz also got tripped up when trying to also name departments he would gut.

“Five major agencies that I would eliminate, the IRS, the department of commerce, the department of energy, the department of commerce and HUD,” Cruz said mentioning Commerce twice.

4. Rand Paul and Marco Rubio battle over military spending

Paul and Rubio have different ideas on military spending -- and they weren’t afraid to make that clear.

“Marco, how is it conservative to add a trillion dollar expenditure for the federal government that you're not paying for?” Paul asked Rubio.

“We can't even have an economy if we're not safe,” Rubio said. “There are radical jihadists in the Middle East beheading people and crucifying Christians.”

“Marco, I don't think we're any safer,” Paul said. “I do not think we are any safer from bankruptcy court.”

5. Rand Paul fact checks Donald Trump on TPP

When Donald Trump was asked what he should do differently on the Trans-Pacific Partnership trade deal, his answer centered around China’s currency manipulation.

“It's a deal that was designed for China to come in, as they always do, through the backdoor, and totally take advantage of everyone,” Trump said. [Currency manipulation] is not even discussed in the almost 6,000 page agreement. Not even discussed,” he said.

“The currency manipulation they don’t discuss in the agreement, which is a disaster,” he said.

But Rand Paul inserted himself into the conversation to make a point.

“We might want to point out China is not part of this deal,” Paul said after Trump had finished his answer.

6. Carly Fiorina Hits Trump Over Foreign Policy with Russia

Donald Trump touted his relationship with Vladimir Putin, saying he “got to know him very well” because they were both on the CBS program “60 Minutes.”

“We were stablemates, we did well that night,” Trump said, but Fiorina hit back saying she knows the real estate mogul “fancies himself a very good negotiator.”

“I accept he's done a lot of good deals. Mr. Trump ought to know we should not speak to people from a position of weakness,” Fiorina said. “One of the reasons I've said I would not be talking to Vladimir Putin right now, although I have met him as well, not in a green room for a show, but in a private meeting,” Fiorina said. “One of the reasons I said I wouldn't be talking to Vladimir Putin right now is because we are speaking to him in a position of weakness brought on by this administration.”

7. Marco Rubio on Putin

After Trump and Fiorina’s back and forth on Putin, Rubio noted: “I never met Vladimir Putin,” but it didn’t stop him from aggressively going after the Russian president. In a way Trump did not.

“I know enough about him to know he is a gangster,” Rubio said. “He is basically an organized crime figure that runs the country, controls a 2 trillion dollar economy and is using to build up his military in a rapid way despite the fact his economy is a disaster.”

The direct fire earned him applause from the debate hall.

8. Ben Carson says 'no problem' about vetting on past

Ben Carson has faced scrutiny over inconsistencies in statements about events in his life, but said during the debate he supports the media vetting presidential candidates.

“The fact of the matter is, what, we should vet all candidates,” Carson said. “I have no problem with being vetted,” he added. “What I do have a problem with is being lied about.”

Carson instead accused Hillary Clinton of lying about whether the Benghazi attacks constituted terrorism.

“When I look at somebody like Hillary Clinton, who sits there and tells her daughter and government official that no, this was a terrorist attack, and then tells everybody else that it was a video, where I came from, they call that a lie,” he said.

Carson also thanked the moderators avoiding questions from many years ago, jabbing at the media at the same time.

“Well, first of all, thank you for not asking me what I said in the 10th grade,” Carson said. “I appreciate that.”

ABC News' Katherine Faulders contributed to this report.