“He’s my man,” says Arizona Republican Sen. John McCain, who won his party’s endorsement in 2008 and knows his way around Iowa and New Hampshire. “He’s a dark horse. Keep an eye on him.”

Graham’s formal launch Thursday of a political committee “Security Through Strength” left some on Capitol Hill scratching their heads. A few of his colleagues chuckled when asked in the marbled hallways what they thought of the defense hawk embarking on a serious presidential campaign.

Graham, who is known as a hardworking legislator, but not a senator with gravitas, is testing the waters at a time when the 2016 field is already packed with potential. The 2016 GOP primary could have a host of past and present GOP governors, from Scott Walker to Mitt Romney to Jeb Bush to Chris Christie. And many of Graham’s colleagues, including Sens. Marco Rubio, R-Fla., Rand Paul, R-Ky., and Ted Cruz, R-Texas, have already been laying the groundwork for their own campaigns for months.