WASHINGTON — A batch of leaked emails obtained by a civil rights advocacy group show that Stephen Miller, the White House adviser with a direct hand in shaping President Trump’s hard-line immigration policies, promoted theories popular with white nationalist groups to an editor at a prominent conservative publication before he joined the administration.

The group, the Southern Poverty Law Center, on Tuesday published a summary of some 900 messages that Mr. Miller sent to Breitbart News from March 2015 to June 2016. The center shared with The New York Times seven pages of the emails included in that summary.

The emails, supplied by Katie McHugh, a former editor at Breitbart, show that Mr. Miller tried to shape news coverage with material he found on at least one website that espouses white nationalist viewpoints, including fringe theories that people of color are trying to engage in “white genocide.” The law center’s investigation, which the group says it will turn into a series, seeks to illustrate how Mr. Miller brought anti-immigrant beliefs to the White House and turned them into policy.

In an exchange from October 2015, when Mr. Miller was an aide to then-Senator Jeff Sessions, Republican of Alabama, Mr. Miller and Ms. McHugh appeared to share a concern that Mexican survivors of Hurricane Patricia, a Category 5 storm, could be granted temporary protected status, or T.P.S., to seek refuge in the United States.