Obama's offensive caught the Romney camp completely by surprise and left them flat-footed and slack jawed.
There, so taken, caught in the act, flat-footed, we are obliged to make our stand.
c.1600, "with flat feet;" meaning "unprepared" is from 1912, U.S. baseball slang, on notion of "not on one's toes;" earlier in U.S. colloquial use it meant "straightforwardly, downright" (1828), from notion of "standing firmly."
Unprepared; surprised •Usu in the phrase catch someone flatfooted: He just stood there flatfooted and watched it roll in (1912+ Baseball)
Straightforwardly; without ceremony: If they were going to turn it down, they would have just flatfooted done it (1828+)