'Throughout his time as Sheriff, Arpaio continued his life's work of protecting the public from the scourges of crime and illegal immigration'.
GOP Rep. Martha McSally is expected to jump in the race as well, which became open after Republican Sen. Flake said on Tuesday about Mr. Arpaio's campaign, "because it won't last long". Democrats in the race include U.S. Rep. Kyrsten Sinema.
Arpaio first announced the news on Twitter, where he has already changed his profile picture to a campaign sign.
Arpaio joins former state Sen. During an interview published on Tuesday, Arpaio reiterated his support for President Donald Trump who pardoned him in August after he allegedly failed to follow court orders and faced six months in prison.
Arpaio flirted with running for governor five times while he was sheriff but ultimately decided against doing so each time.
Flake has been critical of Trump and announced previous year that he would not seek another term.
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When announcing his decision, Flake defended his criticism of Trump.
The lawman known for immigration crackdowns says in a fundraising email that he's concerned about so-called sanctuary cities refusing tougher enforcement of immigration laws. She has told colleagues that she is planning a Senate run but hasn't yet made an announcement.
Arpaio served as the sheriff of Maricopa County that includes Phoenix for 24 years before his crushing 2016 defeat by a little-known Phoenix police sergeant. Kelli Ward, who had attempted to cast herself as the most pro-Trump candidate in the race.
Arpaio, 85, was the long-time sheriff of Maricopa County, Arizona who gained national attention for his tough stances on immigration and his questions about former President Barack Obama's birth certificate, which he claimed was forged.
Arpaio, dubbed "America's toughest sheriff" as a result of his proactive enforcement of illegal immigration, drew media attention after he was pardoned by President Donald Trump previous year.
Arpaio, who campaigned for Trump in 2016, was convicted by a judge who ruled he had willfully violated a 2011 injunction barring his officers from stopping and detaining Latino motorists exclusively on suspicion that they were in the country illegally.