“Our country is at a crossroads facing threats from abroad and attempts from within to restructure the very foundation of our freedoms," Cheney said in a statement.

"America requires leaders to face down these challenges who are confident in who they are and what they stand for. Leaders who know that public service is about something bigger than climbing the political ladder and amassing greater political power."

Cheney also took a shot at Florida Republican Gov. Charlie Crist, who according to reports is mulling an Independent bid for the Senate against Rubio, who he trails badly in GOP primary polls.

"Lately it seems Charlie Crist cannot be trusted even to remain a Republican. I strongly urge him to either stay in the Republican Primary or drop out of the race. The only winners from an independent bid by Crist would be Barack ObamaBarack ObamaOvernight Cybersecurity: Trump tweetstorm on Russia probe | White House reportedly pushing to weaken sanctions bill | Podesta to testify before House Intel OPINION: Trump’s bluff: Perfectly legal Overnight Regulation: Senate Banking panel huddles with regulators on bank relief | FCC proposes 2M fine on robocaller | Yellowstone grizzly loses endangered protection MORE and Harry ReidHarry ReidDems see surge of new candidates Dems to grind Senate to a halt over ObamaCare repeal fight GOP fires opening attack on Dem reportedly running for Heller's Senate seat MORE," Cheney said.

Cheney previously endorsed Trey Grayson over Rand PaulRand PaulOvernight Cybersecurity: Trump tweetstorm on Russia probe | White House reportedly pushing to weaken sanctions bill | Podesta to testify before House Intel Rocky rollout for Senate healthcare bill Overnight Healthcare: Latest on Senate healthcare bill | Four conservatives say they'll oppose | Obama slams bill | Health groups offer scathing criticism MORE in the Kentucky GOP Senate primary.