Sen. Lindsey Graham, about to enter the crowded GOP field of presidential hopefuls, has announced he's retiring from the Air Force Reserve.

The South Carolina Republican, who serves part-time as an attorney in the service, is expected to make his presidential candidacy official on Monday.

Graham has had a long Air Force career, including six and a half years on active duty in the 1980s. He's served in the Reserves since 1995, the year he began his career in the House of Representatives.

Graham noted that he turns 60 this summer, which is the mandatory retirement age for the Reserves.

Graham is one of the Senate's more vocal defense hawks and is expected to make his military and foreign policy credentials a centerpiece of his presidential campaign.