Alaska Gov. Sean Parnell announced Sullivan's departure in a press release on Thursday, including a copy of Sullivan's resignation letter.
"As I explore new opportunities and challenges in the next phase of my life, I intend to seek ways to continue to serve my fellow Alaskans," Sullivan wrote.
Sullivan, who serves in the Marine reserves, recently returned from deployment to Afghanistan. Prior to his most recent position, Sullivan was appointed by then-Gov. Sarah Palin as Alaska's attorney general. He previously served nearly three years as an assistant secretary of State after being appointed by President George W. Bush.
Responding to Sullivan's imminent entrance to the race, Alaska Democratic Party Chairman Mike Wenstrup highlighted the extended GOP primary process that will play out before the eventual nominee can focus on Begich.
“It is going to be a long, contentious year for Joe Miller, Mead Treadwell, and Dan Sullivan — and we still don't even know what Sarah Palin will do," Wenstrup said in a statement.
This is expected to be among the most competitive Senate races of 2014. The race is currently rated Tossup/Tilt-Democrat by Rothenberg Political Report/Roll Call.