(Daniel Acker/Bloomberg)

Stephen Miller, a trusted aide to Sen. Jeff Sessions of Alabama, will join Donald Trump’s presidential campaign on Wednesday, serving as a senior policy adviser to the Republican front-runner.

Hope Hicks, a spokesman for Trump, confirmed Miller’s move on Monday.

[Poll: Donald Trump gained 15 points on Ted Cruz in Iowa in two weeks]

The hiring of Miller is the latest sign that Trump’s tight-knit campaign team is expanding as he continues to lead GOP polls and planning to release more policies in the coming weeks as the primary race unfolds.

It also underscores Trump’s increasingly cozy relationship with Sessions, a prominent immigration hard-liner in Congress who has yet to endorse a candidate.

[“I like him,” Trump says of Sessions]

Months ago, Sessions appeared at a Trump rally in Mobile, Ala., going so far as to don a “Make America Great Again” hat. Now the aide most often at his side is enlisting with Trump.

Miller will work alongside Sam Clovis, Trump’s national campaign co-chairman and chief policy adviser, drafting white papers and assisting the candidate with debate preparations.

The Trump campaign said that Miller will be in Iowa on Thursday to work with the candidate ahead of that evening’s Fox News debate, the final Republican forum before the Iowa caucuses on Feb. 1.

Miller, 30, is a graduate of Duke University who began his career as press secretary for various lawmakers, including former House members Michele Bachmann (R-Minn.) and John Shadegg (R-Ariz.). He began to work with Sessions in 2009 and has been an adviser and confidant to him for the past seven years.

Miller won notice especially in 2013 during the immigration debate when he guided Sessions through countless interviews as the senator opposed the “Gang of Eight” and the push for comprehensive-immigration reform by many Republicans.

When Sessions and Trump began to build a relationship last year, he asked Miller to work with Trump’s campaign as it thought through its immigration position. That experience laid the groundwork for Miller’s hire, a Trump associate said.

Trump’s campaign said Miller plans to suspend his employment with Sessions while he works on the billionaire’s bid.