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.”