Trump's campaign announced the event Wednesday -- a day after the president blamed "both sides" for weekend violence in Charlottesville, Virginia, between white supremacists and counter-demonstrators.
The Aug. 22 rally will take place at the Phoenix Convention Center, the campaign said.
"I am disappointed that President Trump has chosen to hold a campaign rally as our nation is still healing from the tragic events in Charlottesville," Stanton said in a statement Wednesday afternoon. "It is my hope that more sound judgment prevails and that he delays his visit."
The president has been holding campaign-style events in Trump-friendly areas since he took office. Next week's rally will be his first in the West.
Arpaio, 85, is scheduled to be sentenced Oct. 5 and faces up to six months in jail. Attorneys who have followed the case doubt someone his age would be incarcerated, however.
Critics say a pardon would amount to an endorsement of racism.
"If President Trump is coming to Phoenix to announce a pardon for former Sheriff Joe Arpaio, then it will be clear that his true intent is to enflame emotions and further divide our nation," Stanton said.