Senator Jon Kyl announced his resignation Friday from the seat he was appointed to less than four months ago following the death of John McCain, giving Republican Arizona Govenor Doug Ducey a second opportunity to fill the slot.Kyl, 76, had said he was only serving through the end of the year. His resignation is effective Dec. 31 and forces a pivotal decision by the state's Republican governor.That's because voters in 2020 will get to decide who fills McCain's seat for the final two years of its six-year term. Democrats picked up Arizona's other Senate seat in November and are already targeting the state in 2020 as part of their possible path to re-taking control of the Senate, increasing the pressure on Ducey to select someone who can hold the seat for the GOP.Ducey said he will pick a replacement "in the near future." His office has been typically tight-lipped about who might fill the seat, leading to wild speculation in Arizona and Washington. One of the most often-mentioned names is Ducey's chief of staff, Kirk Adams, a onetime state lawmaker who resigned from the governor's office on Nov. 26 and whose last day working for Ducey is Friday.Eileen Klein, whom Ducey appointed as Arizona's state treasurer last year, is another possibility. The most prominent name is Representative Martha McSally, who is fresh off narrowly losing this year's senate race to Democrat Kyrsten Sinema.Democrats are already lining up to challenge whoever is selected. Those testing the waters include U.S. Representative Ruben Gallego, former astronaut and gun control advocate Mark Kelly and former Arizona Attorney General Grant Woods, a onetime McCain chief of staff who became a Democrat this year and a top Sinema booster.Ducey's selection of the well-respected Kyl following McCain's Aug. 25 death allowed him to dodge a possible controversial pick during his own re-election campaign. But Kyl, who had previously served in the Senate for Arizona, made it clear he only agreed to the appointment out of a sense of duty and had no plans to stay in the job for long.Kyl's brief resignation letter said he decided to resign at the end of 2018 so that Ducey's new appointee "can begin the new term with all other senators in January 2019 and can serve a full two (potentially four) years."Kyl noted that when he accepted Ducey's appointment that he agreed to serve through December and then re-evaluate whether to serve longer."Senator Kyl didn't need to return to the Senate," Ducey said in a statement. "His legacy as one of Arizona's most influential and important political figures was already without question. But he did return, and I remain deeply grateful for his willingness to step up and serve again when Arizona needed him. I wish him and his family all the best."McCain died at age 81 at his ranch near Sedona, Arizona, just over a year after he announced he had glioblastoma, an aggressive brain cancer that came with a dire diagnosis.