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Joe Arpaio, the hard-line anti-immigration sheriff from Maricopa County, Ariz., will appear with Donald J. Trump in Iowa on Tuesday and endorse his candidacy, according to Mr. Trump’s campaign.

Mr. Arpaio, who espouses some of the Republican Party’s most conservative views about undocumented immigrants, will appear with Mr. Trump in Marshalltown, a month after appearing with Mr. Trump in Arizona. Mr. Trump has made combating illegal immigration a staple of his candidacy, including a proposal to build a wall at the southern border.

“I have great respect for Sheriff Arpaio,” Mr. Trump said in a statement. “We must restore law and order and respect the men and women of our police forces. I thank him for his support of my policies and candidacy for president.”

In a statement released by the campaign, Mr. Arapaio said: “Donald Trump is a leader. He produces results and is ready to get tough in order to protect American jobs and families. I have fought on the front lines to prevent illegal immigration. and I know Donald Trump will stand with me and countless Americans to secure our border. I am proud to support him as the best candidate for president of the United States of America.”

Mr. Arpaio’s endorsement is part of Mr. Trump’s closing argument ahead of the Iowa caucuses, focusing on immigration, the issue that helped him dominate the political discussion in 2015.

A day earlier, Mr. Trump’s aides announced that Stephen Miller, an aide to Senator Jeff Sessions of Alabama, is joining his team as a policy adviser. Mr. Sessions is a conservative Republican who has spoken out forcefully against a comprehensive immigration reform bill.

But Mr. Sessions has also been cited by Senator Ted Cruz, Mr. Trump’s main opponent in the Iowa caucuses, as he has fended off questions about his own commitment to dealing with the subject of undocumented immigrants.

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