Hoping to build on a recent surge in momentum, state Sen. Scott Brown (R) launched his third ad Sunday in the Massachusetts Senate special election, a 30-second spot in which he seeks to play up his national security credentials.

Brown, a longtime member of the Massachusetts National Guard, goes after his Democratic opponent, state Attorney General Martha Coakley, for supporting conducting trials of accused terrorists in U.S. rather than military courts. Brown doesn’t mention Coakley directly, noting instead that “Some people believe our Constitution exists to grant rights to terrorists who want to harm us. I disagree.”

Despite his underfunded, long-shot candidacy, Brown has been on offense in the general election contest, while Coakley has charted a cautious course.