It's worth keeping in mind that people on Earth DID fight back. Senators were barricading themselves in their offices trying to Rally support from the military and the public, the President's own elite guard were sent to take out the planet's main news outlet *before* they broke ranks and tried getting the truth out.
This was a long time in the planning. Clark and his associates had over a year to get his people into key positions throughout the Alliance. I think the only reason things got as violent as they did was because Sheridan's actions at Jupiter forced them to initiate things a little sooner than they would have liked. This in turn may have caught General Hague and his people a little by surprise, hence them having to shoot their way out of the Sol system.
Lets also remember that there's been a steady anti-alien propaganda war being waged ever since Santiago was assassinated. You keep repeating the big lie often enough, people will start to believe it. And hey, if you're building on a prejudice that already exists, so much the better.
As for the Nightwatch crazies, I think it's pretty obvious that this lot were the hardcore human supremacist types, as opposed to the well meaning but mis-informed like Zack very nearly was. Indeed, I wouldn't be at all surprised if they were originally a part of Home Guard.