The Old Club (or "the CLUB") meet in the Hartleman Club, a sedate place in a quaint little street in the noble quarter, to plot a return to the good old days of the Empire. They're not mad flag-waving nutters, though: they realise that the Emperor is in no fit state to rule (some of their members are influential enough to be privy to that secret) and they know that just raising the old flag and declaring a new Empire will just get the City crushed.
No, they want to go about it the clever way. Supposing disaster were to strike in one of the Four Nations... its government collapses, there's panic in the streets, food is running out... the common folk band together to form an emergency People's Government, which invites in Horizon forces in order to help restore order... and fairly soon, the nation is effectively a protectorate of Horizon. At which point, it no longer becomes possible for the other three Nations to band together and destroy Horizon easily, and thus the seeds of the New Empire will be sown!
The Cause and the Club enjoy good relations; the bulk of the members are of the opinion that both cliques' agendas would be good for Horizon. There's a slight overlap of membership - folk who want to advance both agendas - but most members are of the opinion that turning Horizon into a monarchy is more important than re-establishing the Empire (or vice versa) and therefore are only members of one.