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.