"If you don't think walls work, ask Israel." That's how Republican front-runner Donald Trump defended his immigration plan, specifically to build a wall and deport million of undocumented immigrants. It's not the first time Trump has used Israel's wall as an argument for a wall between the U.S. and Mexico border, but it was the first time that he was publicly challenged by his fellow Republicans. "It's a silly argument. It's not an adult argument," Gov. John Kasich of Ohio, the underdog, said about Trump's extremist immigration plan. Kasich has been a bit of an outsider when it comes to GOP issues. He even took a favorable stance on same-sex marriage in the first debate. And on Tuesday night, he took a stance against Trump, saying that it was ridiculous to think "we are going to somehow pick them [11 million immigrants] up at their house, and ship them to Mexico." Trump was quick to fire back, but Kasich got backup from none other than Jeb Bush, whose campaign relies heavily on his performance in this debate. "It would tear communities apart," Bush said to Trump. He added that "even having this conversation sends signals" to voters, to other countries around the world, but also to Democratic competitors. Bush said "They’re doing high fives in the Clinton campaign when they hear" the GOP arguing over immigration. Trump's response? To spit some one-liners at Kasich and Bush, which received applause from the audience. The only person to back Trump was Texas Sen. Ted Cruz, who used his parents' immigration story as an argument for legal immigration. Trump initially shot to the top of GOP-heap with his harsh –and inaccurate– criticism of undocumented immigrants, particularly people from Mexico, who he said were "bringing drugs. They’re bringing crime. They’re rapists." While his stance has drawn outcrys of racism from the Latino community, it's only helped him in the polls. And other candidates have been all but silent on the issue, until tonight. "False little things don’t work when it comes to the truth. We can’t ship 11 million people," Kasich continued to argue, as Trump shouted over him. "I've built an unbelievable company worth billions and billions of dollars," Trump said, dismissively. "I don't have to hear from this man, believe me."