In an appearance at the Iowa State Fair on Tuesday, Sen. Marco Rubio (R-FL) criticized Donald Trump’s immigration plan—specifically the proposals to end birthright citizenship and deport the millions of illegal immigrants in the U.S.—as “not workable” and “not realistic.”

Instead of ending the automatic grant of citizenship provided by the Fourteenth Amendment to the U.S. Constitution to all children born on American soil, said Rubio, the better solution would be to address the flaws in our immigration system that are abused by illegal immigrants.

“I’m open to doing things that prevent people who deliberately come to the U.S. for the purposes of taking advantage of the 14th Amendment, but I’m not in favor of repealing it,” Rubio told an audience at the Iowa State Fair, according to a report by Politico.

Overall, continued Rubio, Trump’s plan was not feasible and was unlikely to ever become law. “It’s really not a workable plan that could ever pass Congress.”

“Obviously, there are a couple of ideas that he shares with others, but by and large it doesn’t sound to me like a plan that has any chance of passing.” Rubio told the Des Moines Register.

Two years ago, Rubio was the subject of criticism for his support for the Gang of Eight comprehensive immigration bill that passed the Senate but stalled dead in the House, and he has since backed off from the bill. He has said several times on the campaign trail that comprehensive immigration reform is not possible, and the better solution is to pass several smaller bills, starting with border security.

Immigration is “a serious issue,” said Rubio. “We have to address it as a country. But it’s much more complex than people sometimes give it credit for… It does need to be addressed and it does need to be addressed in a series of manners; we’re not going to be able to do it in one big piece of legislation — we learned that two years ago, the last time we tried.”

“We can’t [pass immigration reform] until we prove to Americans that illegal immigration is under control,” said Rubio, who advocated addressing immigration on three fronts. First, “we do need to enforce our immigration laws better. Until we do that, we won’t be able to make progress on immigration.”

Second, continued Rubio, we need to “modernize our legal immigration system toward a merit-based system of immigration that allows us to compete in the global competition for talent.”

And third, “we’ll have to realistically deal with the 12 or 13 million people that are here, in the ways I’ve outlined in the past, allowing people who can pass a background check, and pay taxes and pay a fine, to receive a work permit. And that’s all they’ll receive for a significant period of time.”

He added that “eventually, after a long period of time,” he was open to allowing these immigrants to apply for a green card. “At minimum, we need to deal realistically with what we have now and make sure that it never happens again.”

However, said Rubio, Trump’s proposal to deport the millions of illegal immigrants in this country, was not realistic. “We have 12 or 13 million human beings who have been here for a long time,” he said. “There’s not really a realistic way of rounding up and deporting 12 or 13 million people and our nation wouldn’t want to do that anyway.”

Follow Sarah Rumpf on Twitter @rumpfshaker.