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President Obama on Thursday denounced Donald J. Trump’s campaign promise to deploy a “deportation force” to round up millions of immigrants in the country illegally and send them home, calling it unrealistic and un-American.

“The notion that we’re going to deport 11, 12 million people from this country — first of all, I have no idea where Mr. Trump thinks the money’s going to come from,” Mr. Obama said in an interview with George Stephanopoulos of ABC News. “It would cost us hundreds of billions of dollars to execute that.”

“Imagine the images on the screen flashed around the world as we were dragging parents away from their children, and putting them in what, detention centers, and then systematically sending them out,” Mr. Obama added. “Nobody thinks that that is realistic. But more importantly, that’s not who we are as Americans.”

Mr. Trump’s plan to deport illegal immigrants generated sharp exchanges at the Republican presidential debate on Tuesday night as both former Gov. Jeb Bush of Florida and Gov. John R. Kasich of Ohio dismissed it as unrealistic while Senator Ted Cruz of Texas supported him.

Mr. Trump stuck by his plan in an interview the morning after the debate. “You’re going to have a deportation force and you’re going to do it humanely,” he said on “Morning Joe” on MSNBC, citing deportations under President Dwight D. Eisenhower. “Look, we have to do what we have to do, and Ike did it and other people have done it.”