Republican presidential front-runner Donald TrumpDonald John TrumpEx-ethics chief calls on Trump to end 'monstrous' migrant policies Laura Bush blasts Trump migrant policy as 'cruel' and 'immoral' US denies report of coalition airstrike on Syria MORE is shifting his focus back toward the key GOP issue of immigration amid a slide in polls from Iowa.

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Trump said in an interview broadcast Sunday that retired neurosurgeon Ben CarsonBenjamin (Ben) Solomon CarsonAn ode to fathers Ben Carson walks back plan to triple rent for poor Ben Carson's rent increases can empower domestic abusers MORE, who took the lead in the first-in-the-nation caucus last week, is “very, very weak on immigration” and “believes in amnesty strongly.”

“He’s very, very weak on immigration, and I’m very strong on immigration,” Trump said on CNN’s “State of the Union.” “He believes in amnesty strong. He believes in citizenship – he’s going to give citizenship to people here illegally, and you can’t do that.”

He also said Rep. Paul Ryan (R-Wis.), who’s expected to be the next Speaker, is “very weak on immigration.”

“I think he’s a very nice guy, I think he’s a very capable guy, but there’s a couple of issues he’s very weak on. And one of those issues is immigration,” Trump said. “We have to have a border. We have to have a wall. We’re going to have a wall.”

Trump additionally went after fellow presidential candidates Jeb Bush and Sen. Marco Rubio (R-Fla.) for their stances on issue.

“But I would tell Carson and I would tell other people that are very weak on it – Jeb happens to be. Rubio happens to be very weak on immigration,” Trump said. “He was a member of the gang of eight, they wanted to give whole country away, Rubio. Then his polls went down on and all of a sudden he got out of the gang of eight pretty fast.”

Trump also said the temporary worker program needs to be reformed to ensure people don’t overstay their visas.

“People with visas – they come back right in. In other words, people can come back in with visas,” Trump said.

“Look – we need to have a country or not? These are illegal people,” he added.

--This report was updated at 9:55 a.m.