My favorite tactic when I want to attack a civ after Nationalism is to do the following:
1.)If possible, have an MPP with EVERYONE besides the civ you want to eliminate.
2.)Declare war on the civ you want to attack, but don't attack it at all. instead, if possible, place a worker or something irresistable for the AI to attack (workers work best, AI is hard-wired to capture undefended workers at all costs).
3.)AI captures the worker and EVERYONE else declares war on that one civ (and most often, every AI civ will be gracious with you except the one you're at war with). It's a great (if cheesy) way to seal up diplo victories (by causing the entire world to be at war with your arch-rival while you build the UN and hold the vote), or just to cause trouble.
And if the civ you're fighting has access to bombers, you often don't need the worker bait, you can just let your territory be bombarded. As long as you are on the business end of the first shot, the world will rally to your cause. And the best part? Once the MPP runs out and if that civ is still alive, you can cancel the MPPs and extract yourself from the war while the AIs will continue to fight to the death and pull themselves into Communism or Monarchy while you can remain a Democracy (and pull away with the tech lead). Or you can continue to play the AIs against each other for small gains while you consume the entire world.
It's kinda cheap, but playing mind games with the AI can be pretty fun.