Sen. John McCain's brain cancer diagnosis raises the question of what would happen if he were unable to carry out his duties in the U.S. Senate.
It could trigger a political free-for-all next year.
Under Arizona law, if McCain were no longer able to serve, the governor must appoint a replacement who has to be from the same party.
The successor would serve until the next statewide election. The winner of that election would serve out the unexpired term, which ends in 2022.
The law would allow Gov. Doug Ducey to appoint himself to fill the seat, according to legal experts.
MORE: McCain's office says he has cancerous brain tumor
More likely, political analysts say, is that Ducey would name a "placeholder" -- someone to hold the seat until the special election, and then bow out.
McCain's departure would set up the possibility of two U.S. Senate elections in Arizona in 2018: the McCain seat and Republican Sen. Jeff Flake's defense of his seat.
That could give Arizona voters the power to decide which party controls the U.S. Senate.
Since U.S. Senate seats don't come open very often in Arizona -- just 12 men have held the two seats since statehood 105 years ago -- and the terms are six years, expect several members of Congress to consider a run.
McCain's office is optimistic of his return to his duties, saying in Wednesday night's statement, "Further consultations with Sen. McCain's Mayo Clinic care team will indicate when he will return to the United States Senate."
The tumor, a glioblastoma, is a highly malignant, aggressive, cancerous tumor, according to the American Brain Tumor Association. McCain is 80 years old. He is getting treatment, including chemotherapy and radiation, at the Mayo Clinic.
PHOTOS: Sen. John McCain through the years
Sens. Kelly Ayotte (R-NH)(C), Lindsey Graham (R-SC) (R) and John McCain (R-AZ), chairman of the Senate Armed Services Committee, hold a news conference at the U.S. Capitol Feb. 24, 2016 in Washington, DC. (Photo: Chip Somodevilla/Getty Images)
Photo taken in 1967 shows John McCain in a Hanoi hospital as he was being given medical care for his injuries. McCain was captured in 1967 at a lake in Hanoi after his Navy warplane was downed by the Northern Vietnamese army. (Photo: Getty Images)
A photo taken 26 October 1967 shows US Navy Airforce Major John McCain being rescued from Hanoi's Truc Bach lake by several Hanoi residents after his Navy warplane was downed by Northern Vietnamese army during the Vietnam War. (Photo: Getty Images)
A photo taken in 1967 shows US Navy Airforce Major John McCain being examined by a Vietnamese doctor. (Photo: Getty Images)
Undated picture shows US Representative Pete Peterson (L), US Senators John McCain (R) and John Glenn (rear) leaving Hanoi's central prison. Once nicknamed the Hanoi Hilton, it is where McCain and Peterson were detained. (Photo: Getty Images)
U.S. Senator John McCain holds up photos of himself, as a 30-year-old man wounded and captured in 1967 in North Vietnam, outside the Army Museum in Hanoi 19 October, 1992. (Photo: Getty Images)
Hanoi Army Museum Director hands to U.S. Senator John Kerry the flying helmet of U.S. Senator John McCain, who was a U.S. Navy pilot and was shot down over Hanoi in 1967. (Photo: Getty Images)
Sen. John McCain, a former POW, looks at a display of personal belongings of American POWs at the joint POW/MIA archives center in the Hanoi Army Museum in 1993. (Photo: Getty Images)
US Sen. John McCain, R-AZ., and his wife Cindy (L) attend a MIA remains repatriation ceremony 11 April 1995 at Hanoi's airport. (Photo: Getty Images)
John Kerry and John McCain examine 24 June 1992 a book containing the names the 'total unaccounted for' soldiers who served in the Vietnam War. (Photo: Getty Images)
John Kerry displays a report 13 January 1993 concluding a year-long investigation, that rejected allegations that Americans were 'knowingly abandoned' by US officials. (Photo: Getty Images)
Bill Clinton talks to John McCain after announcing that the US is establishing normal relations with communist Vietnam on July 11, 1995. (Photo: Getty Images)
Republican presidential candidate US Senator Bob Dole holds a press conference along the border of the United States with Mexico near San Diego, California 22 March, 1996. (Photo: Getty Images)
US Republican presidential candidate Bob Dole and US Senator John McCain wave to members of the American Legion at the group's annual convention, Sept. 1996. (Photo: Getty Images)
Republican Senator John McCain announces his official bid for the 2000 White House race 27 September 1999 in Nashua, New Hampshire. (Photo: Getty Images)
Republican presidential hopefuls from left, Utah Senator Orrin Hatch, financier Steve Forbes, Arizona Senator John McCain and radio commentator Alan Keyes wait for the GOP debate to commence November, 1999. (Photo: Getty Images)
Gary Bauer, Texas Gov. George Bush, Steve Forbes, Senator Orrin Hatch, Alan Keyes, and US Senator John McCain, pose for photographers December, 1999 just before participating in a television debate for Republican candidates. (Photo: Getty Images)
US President George W. Bush (R) is greeted by US Senator John McCain, Republican of Arizona, after arriving at Phoenix Sky Harbor Air Port 26 March, 2004. (Photo: Getty Images)
Sen. John McCain gestures after talking to a group of Bush/Cheney supporters on the day before the first-in-the-nation New Hampshire primary January 26, 2004. (Photo: Getty Images)
US Senators Hillary Rodham Clinton, Susan Collins and John McCain in Estonia in 2004. (Photo: Getty Images)
US President George W. Bush, and US Senator and former election campaign rival, John McCain walk to the Bush-Cheney campaign bus August 2004. (Photo: Getty Images)
Sen. Elizabeth Dole discusses with Sen. John Warner and Sen. John McCain May 13, 2004 prior to a hearing on the contingency reserve fund. (Photo: Getty Images)
Senator John McCain (R-AZ) speaks at a memorial service held by the family of Cpl. Pat Tillman for Tillman, who was killed in action in Afghanistan April 22, 2004. (Photo: Getty Images)
US Senator John McCain addresses the Republican National Convention at Madison Square Garden in New York City August 2004. (Photo: Getty Images)
Delegates wave placards of support for US Senator John McCain at Madison Square Garden in New York City August, 2004.(Photo: Getty Images)
Sen. Lieberman unveils recommendations made by the 9/11 Commission covering subjects ranging from intelligence reform, border and transportation security, information sharing and outreach to the Muslim world. (Photo: Getty Images)
U.S. Senator John McCain makes a guest appearance on 'The Tonight Show with Jay Leno' November 4, 2004. (Photo: Getty Images)
Gov. Arnold Schwarzenegger speaks as Senator John McCain looks on at a campaign stop press conference where McCain announced his endorsement of Schwarzenegger's Special Election ballot measures on October 10, 2005 (Photo: Getty Images)
Immigrants participate in an interfaith prayer service on the west lawn of the U.S. Capitol to protest current measures that criminalize clergy and social service providers who give aid to undocumented immigrants March 27, 2006. (Photo: Getty Images)
U.S. Sen. John McCain and actor Will Smith attend a cocktail party for Time Magazine's 100 Most Infuential People issue at Jazz at Lincoln Center May 8, 2006. (Photo: Getty Images)
Republican presidential hopeful Rudy Giuliani listens as John McCain speaks during the Republican Presidential Candidates Debate May 2007. (Photo: Getty Images)
Republican presidential hopefuls stand at their respective podiums during the South Carolina Republican Presidential Candidates Debate May 2007. (Photo: Getty Images)
John McCain and Barack Obama testify during a Senate Homeland Security and Governmental Affairs Committee on Capitol Hill July 18, 2006. (Photo: Getty Images)
John McCain speaks during a rally April 25, 2007 in Portsmouth, New Hampshire .McCain officially announced his 2008 bid for the presidency with a campaign rally. (Photo: Getty Images)
Brad Henry of Columbia, South Carolina puts up a John McCain campaign sign in front of the Koger Center already adorned with other signs May 15, 2007. (Photo: Getty Images)
Republican presidential candidate John McCain his wife Cindy and his vice presidential running mate Alaska Governor Sarah Palin campaign outside Tom's Dinner in Pittsburgh, Pennsylvania on August 30, 2008. (Photo: Getty Images)
Presumptive Republican presidential nominee John McCain stands with Alaska Gov. Sarah Palin onstage at a campaign rally August 29, 2008. (Photo: Getty Images)
Republican presidential hopeful U.S. Sen. John McCain (R-AZ) speaks at a campaign event at Pelhan High School December 19, 2007. (Photo: Getty Images)
Senate Minority Leader Mitch McConnell (R-KY) (L) listens wile Sen. John McCain (R-AZ) (R) speaks as President Barack Obama meets with Senate Democrats on Capitol Hill December 6, 2009 in Washington, DC. (Photo by Brendan Smialowski/Getty Images)
U.S. Sen. John McCain (R-AZ) speaks as his wife Cindy listens during a visit to a campaign office one day before Arizona's primary election August 23, 2010 in Phoenix, Arizona. (Photo by Eric Thayer/Getty Images)
Senate Armed Services Committee ranking member U.S. Sen. John McCain (R-AZ) talks with reporters as he leaves the weekly Senate Republican policy luncheon at the U.S. Capitol Nov. 29, 2011 in Washington, DC. (Photo: Chip Somodevilla/Getty Images)
U.S. Senator John McCain of Arizona flips the coin in the air before the Kraft Fight Hunger Bowl between the Arizona State Sun Devils and the Navy Midshipmen at AT&T Park on December 29, 2012 in San Francisco. (Photo by Ezra Shaw/Getty Images)
U.S. Sen. John McCain (R-AZ) holds up his smart phone to show he is not playing poker before the Senate Foreign Relations Committee vote on a resolution on Syria on Capitol Hill September 4, 2013 in Washington, DC. (Photo: Mark Wilson/Getty Images)
US Senator John McCain waves to the media as he leaves the Ministry of External Affairs in New Delhi on July 2, 2014. (Photo: PRAKASH SINGH/AFP/Getty Images)
Chairman Sen. John McCain (R-AZ)(L) talks with U.S. Sen. Jack Reed (D-RI) before the start of a Senate Armed Services Committee hearing on Capitol Hill, January 21, 2015 in Washington, DC. (Photo by Mark Wilson/Getty Images)
Sen. John McCain (R-AZ) arrives for work on Capitol Hill hours after voting NO on the GOP 'Skinny Repeal' health care bill, on July 28, 2017 in Washington, DC. (Photo: Mark Wilson/Getty Images)
Sen. John McCain (R-AZ) leaves the the Senate chamber at the U.S. Capitol after voting on the GOP 'Skinny Repeal' health care bill on July 28, 2017 in Washington, DC. (Photo: Justin Sullivan/Getty Images)
Sen. John McCain (R-AZ) receives the the 2017 Liberty Medal from former Vice President Joe Biden at the National Constitution Center on Oct. 16, 2017 in Philadelphia. (Photo: William Thomas Cain/Getty Images)
Sen. John McCain (R-AZ) moves through the U.S. Capitol in a wheelchair November 30, 2017 in Washington, DC. (Photo by Chip Somodevilla/Getty Images)