Sen. John McCainJohn Sidney McCainRubio asks Army to kick out West Point grad with pro-communist posts The VA's woes cannot be pinned on any singular administration Overnight Defense: Mattis offers support for Iran deal | McCain blocks nominees over Afghanistan strategy | Trump, Tillerson spilt raises new questions about N. Korea policy MORE is facing a GOP primary challenge next year from Chris Simcox, the co-founder of the anti-illegal immigrant Minutemen group.

Simcox criticized the Arizona Republican for his position on immigration in a statement announcing his candidacy.

"John McCain has failed miserably in his duty to secure this nation's borders and protect the people of Arizona from the escalating violence and lawlessness," he said.

"He has fought real efforts over the years at every turn, opting to hold our nation's border security hostage to his amnesty schemes," he went on. "Coupled with his votes for reckless bailout spending and big government solutions to our nation's problems, John McCain is out of touch with everyday Arizonans. Enough is enough."

McCain, of course, previously worked to pass legislation with Sen. Ted Kennedy (D-Mass.) on immigration reform. McCain was criticized by the more conservative wing of the GOP during his presidential run for his position on the issue.