By planning a rally and reportedly stating that he is "seriously considering" a pardon for Arpaio, Trump is seen as attempting to change the subject and perhaps to further unite his base of support in the face of criticism.
At the rally, Trump is also expected to speak in favor of a candidate he's backing in the upcoming midterm elections; on Thursday, he tweeted his support for Kelli Ward, the challenger to Republican Sen. Jeff Flake. In recent interviews and in his new book, Flake, who refused to vote for Trump, said Republicans made a "Faustian bargain" by backing him.
Arpaio, 85, was voted out of the Maricopa County office in November; last month, he was found guilty of criminal contempt by a federal judge in Arizona over his refusal to stop targeting Latinos for more than a year after a court order barred his deputies from detaining immigrants.
Arpaio is facing a potential six-month prison term; he's not scheduled to be sentenced until Oct. 5.
Trump and Arpaio have often spoken of their support for each another; the former sheriff backed Trump's presidential campaign and spoke at events on his behalf.
News of the Phoenix rally came via Twitter on Wednesday night, when Trump posted a link to his campaign's site where prospective attendees could register for tickets.
In a statement about the planned Trump rally, Stanton said, "It is my hope that more sound judgment prevails and that he delays his visit."
He added that his city will prepare to protect First Amendment rights and the public's safety if the rally goes ahead as planned.
