Sen. Jon Kyl, who was appointed to represent Arizona after Sen. John McCain died of cancer in August, will step down at the end of the year, Gov. Doug Ducey announced Friday.

When Kyl was tapped in September to fill the seat, he promised only that he would serve until the end of the year. Ducey, a Republican, will now have to find another replacement to serve until a special election in 2020.

The 76-year-old Kyl, who previously served in the Senate for nearly two decades, retired in 2013 after rising to become the second-highest-ranking Republican senator. As Slate’s Josh Voorhees wrote at the time of Kyl’s appointment, Ducey’s decision seemed a smart one: Kyl was a reliable vote for Brett Kavanaugh, and after a year, Kyl could return to his high-paying job at the law firm Covington & Burling, allowing Ducey the opportunity to nominate an up-and-comer of Arizona politics. The second replacement could then use a couple of years to build up his or her name recognition and campaign funds in order to secure the seat in 2020.

Kyl, in his resignation statement, indicated the move was meant to facilitate such a plan (though it is not clear if he intends to return to his law firm). “When I accepted your appointment, I agreed to complete the work of the 115th Congress and then reevaluate continuing to serve,” he said in the statement. “I have concluded that it would be best if I resign so that your new appointee can begin the new term with all other Senators in January 2019 and can serve a full two (potentially four) years.”

Who this second replacement will be is still unknown. Martha McSally, who narrowly lost the state’s other Senate seat this year to Kyrsten Sinema, had been considered a leading contender. But the Washington Post reports that Ducey has “lost enthusiasm” for McSally, in part because of a memo her team circulated that blamed her defeat on external factors, rankling Ducey and other Republicans in the state. According to the Arizona Star, Senate Majority Leader Mitch McConnell has expressed “intense interest” in McSally, but Ducey will have to balance pressure from those in the White House and on Capitol Hill with the advice of those in his own circle and that of the other political figures in the state. There are few obvious alternatives to McSally, and according to the Post, Kirk Adams, Ducey’s outgoing chief of staff, is considered a serious contender.