The Party

The Party started out as a loose coalition of Nobles, industrialists and military officers united by a common goal: they wish to see the Imperium rule the sector and destroy all Daemons, Witchs, and Xenos in the sector. The reasons and rationalizations varied, but the central principle remained the same: the need to have a single unified body to guide the Imperium.
The Party mostly remains a coalition of Nobles, industrialists and military officers. Nobles and those with military leanings now make up the majority of the Party's upper membership. The Party has a support organization separate from its political wing, which mainly deals with business matters such as fundraising or advertising. These are overseen by the CEO, along with her executive. The CEO of the Party is not involved in the political wing, and takes orders from Party members with the highest positions in Parliament.
As they have control of the Agrippa Sector, the Governor-Militant and his cabinet also has supreme authority within the Party. The Party is, by definition, Fascist. They are extremely right-wing and claim that the Imperium is a 'natural community,'. This, of course, proves that the Imperium is the rightful, predestined ruler of mankind. Loyalty to the Imperium(and hence, to the Party) is portrayed as transcending all other loyalties. The Party has no qualms about using whatever means are necessary to further what they see as a higher purpose. The Party's ideals of youth and virility have also caught on among the people, and most celebrities now embody these ideals. Much of this comes from the media image of the Governor-Militant, who is portrayed as the pinnacle of both.
Externally, the Party is populist, almost to an extreme. They seek to provoke action in the people of the Agrippa Sector against perceived threats or enemies, real or invented. Currently, this is focused on the retaking of the world’s lost to the forces of Chaos, the extermination of the Tau and "crimes against the people." Internally, however, the Party is elitist: members treat other Party members as a select group from which authority trickles downward to Party supporters, and then to the common people. They apply this principle when getting supporters to organize 'grassroots' movements and support groups to guide the actions and choices of the masses. The Party treats its enemies with contempt, using intimidation, and violence. It has easy moved the Agrippa Sector into a true totalitarian state to fit its vision. 
Factions
Like any political movement, the Party has its share of fanatics, hangers-on, celebrities, businessmen, and fringe members. Those affiliated with corporations that profited from war and those with direct military ties are currently the powerhouses in The Party.