Republican presidential candidate Ben Carson knocked rival Donald Trump’s immigration plan on Tuesday, saying that deporting the country’s undocumented immigrants “sounds really cool” but would be far too costly and complicated.

“It sounds really cool, you know, ‘Let’s just round them all up and send them back,'” Carson said. “People who say that have no idea what that would entail in terms of our legal system, the costs – forget about it. Plus, where you gonna send them? It’s just a double whammy.”

Carson has been gaining on Trump, the GOP frontrunner, in recent polls. An NBC/Marist poll released Sunday found Carson has jumped to second place in Iowa among Republican voters, seven points behind the real estate mogul.

Trump’s immigration plan calls for deporting the estimated 11 million undocumented workers currently in the U.S., a proposal that opponents say would cost billions and would be nearly impossible to execute. Carson said that he instead favors a plan that would allow undocumented workers the chance to become guest workers.