Prolonging the uncertainty over who will fill the late John McCain’s U.S. Senate seat, the Governor of Arizona on Tuesday announced the appointment of former senator Jon Kyl, but said he has committed to serve only until the end of the year.

Mr. Kyl, a Republican who retired from the Senate in 2012 to become a lobbyist and spend more time with his family, is currently shepherding U.S. President Donald Trump’s nomination of Brett Kavanaugh to the U.S. Supreme Court. Mr. Kyl’s appointment will make it possible for him to vote for the nomination.

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Jon Kyl, right, and Arizona Republican Gov. Doug Ducey at the news conference where it was announced that Kyl would replace the late John McCain in the U.S. Senate until Dec. 31.

Ross D. Franklin/The Associated Press

It’s unclear what more Mr. Kyl, 76, will do in the Senate. He said he agreed to serve briefly out of a “sense of duty,” and will not run for the seat in 2020, when Arizona voters will decide who fills the seat through 2022. The seat will then be up again for a full six-year term.

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Governor Doug Ducey, also a Republican, said Mr. Kyl was “the best possible person, regardless of politics” for the job, and he hopes Mr. Kyl serves past the end of the year.

If Mr. Kyl does leave the Senate after the end of the current congressional term next Jan. 3, Arizona’s governor can appoint another senator for the remaining year. By state law, the senator will have to be a member of the same party as the departing one, in this case the GOP.

For now, Mr. Kyl will pad Republicans’ margin in the narrowly divided Senate. They hold a 51-49 majority, but that dwindled to a single vote while Mr. McCain stayed in Arizona for much of this year being treated for the brain cancer that killed him on Aug. 25.

The GOP is hoping Mr. Kyl will be a more reliable partisan vote than Mr. McCain, whose opposition to a partial repeal of former president Barack Obama’s health-care law pitched the party into turmoil last year.

Mr. Kyl is well respected in Arizona and has been able to avoid many of the battles with activists that complicated Mr. McCain’s career and that of the state’s other senator, Jeff Flake, who is retiring because his feud with Mr. Trump made his re-election impossible.

Mr. Kyl’s entire career in Washington overlapped with Mr. McCain’s, and he served with the state’s senior senator for three terms before stepping down. Mr. Kyl carved out a profile as a reliable conservative vote and a foreign-policy expert.

Mr. McCain’s widow, Cindy, tweeted: “Jon Kyl is a dear friend of mine and John’s. It’s a great tribute to John that he is prepared to go back into public service to help the state of Arizona.”

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Doug Cole, a veteran Republican consultant and former McCain aide, said Mr. Kyl was a safe pick.

“I think McCain would be very happy with the pick. Honours his legacy while putting some major horsepower for Arizona in the seat, at least for now,” he said.

An attorney, Mr. Kyl speaks in a formal, reserved manner that is a sharp contrast to Mr. Trump, and he belongs to an older, less populist vanguard of the GOP. At a news conference where he was introduced as Mr. McCain’s replacement, Mr. Kyl said in a response to a question that he’s only met Mr. Trump once, although he noted he is working for the White House by serving as Mr. Kavanaugh’s “sherpa.”

With a nod toward Mr. McCain, Mr. Kyl said the President’s “desire to jump into the middle of, or be in the middle of a fight – and by the way that reminds me of somebody – can be detrimental in the end to what he’s trying to achieve.”

During his time back in the Senate, Mr. Kyl will have the opportunity to vote on several issues that have been involved in his lobbying activities, from immigration to national security.

Mr. Kyl has lobbied on behalf of young immigrants brought to the United States illegally as children, often called Dreamers, whose fate is in limbo after Mr. Trump tried to end an Obama-era program protecting them from deportation and Congress deadlocked on a solution.

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Mr. Kyl will likely also vote on bills containing tens of billions of dollars for Defence Department programs after he represented defence giant Northrop Grumman. His work was primarily focused on tax issues affecting the company, but Mr. Kyl’s firm, Covington & Burling, also lobbied the House and Senate to drum up support for the B-21 bomber, a stealth aircraft Northrop Grumman is building for the Air Force.

The unusual timing of Mr. Kyl’s appointment could work out for another Arizona Republican who wants to become a U.S. senator – Representative Martha McSally.

She’s currently locked in a tough campaign against Democratic Congresswoman Kyrsten Sinema for the Senate seat Mr. Flake is vacating. Should she lose, Ms. McSally – a former Air Force colonel who once worked on Mr. Kyl’s staff – could be appointed to the Mr. McCain seat if Mr. Kyl leaves it in January.