PHOENIX -- Joe Arpaio, the former Arizona sheriff who was spared a possible jail sentence when U.S. President Donald Trump pardoned him for disobeying a judge, announced Tuesday that he would run for the Senate seat being vacated by fellow Republican Jeff Flake.

The 85-year-old longtime lawman said the lack of support for Trump's agenda in Washington inspired him to make the bid. He also cited supporters who urged him to seek public office again after a crushing 2016 re-election defeat following six terms as sheriff of metro Phoenix. Then Trump offered the pardon last summer.

"If I go to my grave, I don't think I'd be happy if I didn't take the shot to run," Arpaio said, adding that Trump had not asked him to run.

Arpaio's plan could set up a race in which one of the president's most prominent supporters attempts to take over for one of his fiercest critics.

Flake has sparred with Trump over free trade, immigration reform and opening relations with Cuba, even while supporting parts of the president's agenda, like recent tax cuts. Trump, in turn, has denounced the senator, who is not seeking re-election after acknowledging that he could not win a GOP primary in the current political climate.

Arpaio's announcement also raised questions about whether he was serious about the Senate or if he was mainly seeking publicity. Over the years, he flirted with running for Arizona governor no fewer than five times before demurring and abandoning the idea.

Asked whether the White House supports Arpaio's candidacy, spokeswoman Sarah Huckabee Sanders declined to comment.

Arpaio said he would accept a Trump endorsement, but he would not seek it.

"If you know my history, you know they (other candidates) ask for the endorsements," Arpaio said. "You never see me with a list of endorsements."

The former sheriff's ambitions also sparked speculation that he could edge out a former state senator, Kelli Ward, in the GOP primary and could potentially create an opening for Republican Rep. Martha McSally, who colleagues have said is planning a Senate run but has not yet made an announcement.

David Berman, a senior research fellow at Arizona State University's Morrison Institute of Public Policy, said if Arpaio follows through on his announcement, his candidacy will likely hurt Ward's chances.

Arpaio will probably siphon off support from some Trump voters and tea party supporters. "I think he would wipe her out," Berman said.

Zachery Henry, a spokesman for Ward's campaign, said the campaign is not concerned that Arpaio would split the GOP vote to Ward's detriment.

Ward, who lost a 2016 GOP primary challenge to Sen. John McCain, has been endorsed by former Trump strategist Steve Bannon in her campaign to replace Flake. Trump made a favourable tweet about Ward but never formally endorsed her.

The primary to decide nominees will be in August, followed by the general election in November.

For decades, Arpaio was known for jailing inmates in outdoor tents during triple-digit heat and forcing them to wear pink underwear. He also conducted dozens of immigration crackdowns over a nine-year period, retaliated against political enemies and failed to adequately investigate more than 400 sex-crimes complaints made to his office.

Last year, he was convicted of criminal contempt of court for intentionally disobeying a 2011 court order in an immigration case.

His re-election defeat came amid a crush of criticism over the $141 million in legal costs that Maricopa County taxpayers footed for defending him in lawsuits focused on his immigration policies, the deaths of inmates in his jails and a child sex abuse case that was botched by his department's investigators.

Arpaio endorsed Trump in 2016 and appeared alongside him at campaign appearances, including a large outdoor event in the lawman's hometown of Fountain Hills, outside Phoenix.

Critics complained that the media-savvy lawman repeatedly stoked speculation about running for governor to stir publicity for himself and raise money for his re-election campaign as sheriff.

If Arpaio had decided to run for governor, Arizona law would have required him to resign immediately as sheriff.