(CNN) As Donald Trump pursues a purge of the Department of Homeland Security amid what he is promising will be a tougher immigration policy, attention has, again, turned to Stephen Miller -- one of the President's top aides. Miller is one of the youngest and least high-profile members of Trump's inner circle, but on immigration has long been a leading advocate of a strong hand as it relates to the handling of undocumented immigrants trying to enter the United States.

To get a fuller perspective on Miller and his influence on the President, I reached out to CNN White House correspondent -- and rabid Alabama fan! -- Kaitlan Collins . Our conversation, conducted via email and lightly edited for flow, is below.

Cillizza: What is the key to Miller's success -- and longevity -- in this White House and with this President?

Collins: Our reporting shows Miller has already had a lot of success in serving the President. He's become one of the President's closest advisers, and while he may not be victorious with everything he pushes, he certainly has Trump's ear when it comes to immigration.

Take for example what happened last week: Ron Vitiello was well on his way to becoming the director of Immigration and Customs Enforcement when Miller went to the President and told him Vitiello wasn't in favor of him closing the southern border, as Trump had threatened to do for a week. Miller knew how the President would react to this. The White House quickly pulled the nomination -- so quickly that DHS officials thought it was a clerical error until Friday morning. The Homeland Security secretary was never officially informed about the withdrawal. And [Wisconsin Sen.] Ron Johnson, a Republican who chairs the Homeland Security panel, said he was "completely surprised" by it.

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