Service officials said they were looking into whether Cpl. Jesse Thorsen, an Iowa-based reservist, violated the military's rules concerning troops' political participation, Stars and Stripes reported Wednesday.
During the Iowa caucuses Tuesday, Thorsen, in the service for 10 years, spoke in support of Paul's campaign in an interview with CNN. At a post-caucus rally, Paul identified Thorsen as a soldier who "has been with us in our campaign for a few years," and invited Thorsen on stage.
Regulations say troops may attend political events as spectators but not in uniform. The rules bar participation in "partisan political" events and mandate that service personnel avoid media interviews in which partisan views could be advocated, Stars and Stripes said.
Penalties could range from a letter of reprimand to a reduction in rank or dismissal.
Joel Arends, executive director of Veterans for a Strong America, a political action organization, said Thorsen's decision to wear his uniform was "irresponsible." He also criticized the Paul campaign for either not knowing military rules or ignoring them.
"We need troops and veterans at the table, and we need them to be part of the election process," Arends said. "But we don't need troops to be violating regulations. It's all about common sense, and most troops understand that we cannot mix the use of a military uniform with political campaigns."
Stars and Stripes said efforts to reach Paul's campaign for comment were unsuccessful.