It doesn't hurt that most of the kids who are reaching voting age right now remember that time ten years ago when Earth was almost blown up by genocidal aliens.
Earth is a "great power" but the weakest of them. The Centauri would kick their asses. The Minbari already have kicked their asses. They're not even a speed bump to the Vorlons. Isolationism, spying, and extreme military buildup make sense to the person on the street. Political officers aren't a bad idea when a single loose-cannon captain can accidentally get Earth blown up by making one wrong move.
The Earth-Minbari war was started by a single glory-hound officer who chose to disobey orders and then chose to fire on an alien ship without confirming it's intentions. If there had been a political officer onboard it never would have happened.
Sheridan almost started a war with the Centauri at the end of season 2. If the Centauri had pressed the matter, Earth would have been bombarded with asteroids just like Narn was, and humanity would have been enslaved. Playing chicken when you're on equal footing is one thing. Playing chicken when you can't win and the other side knows that you can't win is a whole other level of stupid. Humanity is extremely lucky that the Centauri didn't want to risk extending themselves so far so fast.
Plenty of Clark's decisions make very good sense.