A reader corrects me vigorously:

Jonah–
Strictly speaking, your observation is incorrect:
Penny Johnson (Condoleezza Rice) appeared in multiples episodes of DS9 and an episode of TNG.
Gregory Itzin (John Ashcroft) appeared in multiple episodes of Star Trek: Enterprise, and DS9, and an epidode of Star Trek:Voyager.
Stephen Macht (Paul Wolfowitz) appeared in multiple episodes of DS9.
Lawrence Pressman (Dick Cheney) appeared in several episodes of DS9.
Scott Alan Smith (Ari Fleisher) appeared in an episode of Star Trek: Voyager.
Allan Royal (Karl Rove) appeared in multiple episodes of Star Trek: Voyager.