Arizona Gov. Doug Ducey will have big shoes to fill as he mulls over who he will appoint to temporarily fill the U.S. Senate seat of the late John McCain until a special election can be held to complete McCain's term.

McCain, 81, lost his year-long battle with brain cancer Saturday, leaving behind a decades-long legacy in politics which included presidential runs.

Even as he took a months-long hiatus from Washington to recover in his home state, McCain maintained a voice in Congress, sharing his opinion on legislation and frequently criticizing President Trump’s agenda.

McCain was one of the first to express opposition to Trump’s first presidential pardon in August 2017 of Joe Arpaio — the former sheriff of Arizona’s Maricopa County who was found guilty of a misdemeanor contempt-of-court charge in a trial prosecuted by the Justice Department.

A week earlier, McCain slammed Trump’s remarks surrounding last year's violence in Charlottesville, Va., writing in a tweet there’s "no moral equivalency between racists & Americans standing up to defy hate & bigotry."

In September 2017, McCain shocked his Republican colleagues with a thumbs-down vote against a replacement for "Obamacare," the health care law approved under President Barack Obama.

“I cannot in good conscience vote for the Graham-Cassidy proposal. I believe we could do better working together, Republicans and Democrats, and have not yet really tried,” McCain wrote, in part, after his dramatic vote. “I take no pleasure in announcing my opposition. Far from it. The bill’s authors are my dear friends, and I think the world of them. I know they are acting consistently with their beliefs and sense of what is best for the country. So am I.”

Trump signed a military policy bill in August named for McCain, but in a sign of their testy relationship the president made no mention of McCain's name in remarks at a signing ceremony.

Here’s what happens to Mccain’s Senate seat, according to Arizona election law.

Governor appoints a new senator

Arizona is one of 36 states where a governor makes an appointment to fill a U.S. Senate vacancy, according to the National Conference of State Legislatures. Therefore, Gov. Ducey will appoint an interim senator to fill McCain’s seat.

Because McCain was a Republican, state law requires Ducey to appoint a member of the same party — a move that’s critically important for the Trump administration, as the GOP currently holds a bare 51-49 Senate majority.

The newly appointed senator would be in office until the next general election in November 2020. The interim senator would not be obligated to run in that election.

Whomever is elected to the Senate seat in November 2020 would complete McCain's term, which expires in January 2023.

What happens if the governor chooses a member of Congress?

If Ducey chooses one of the state’s current congressional members to fill the seat, then a special election would need to be held to fill that empty spot. According to the Arizona State Legislature, that election would have to be held “not less than 120 nor more than 133 days” after the vacancy occurs.

Fox News’ Kaitlyn Schallhorn and the Associated Press contributed to this report.