A professional politician is a professionally dishonorable man. In order to get anywhere near high office has to make so many compromises and submit to so many humiliations that he becomes indistinguishable from a streetwalker. —H.L. Menchen
As a member of a legislative body, it is nearly impossible for an individual politician to advance their legislation by themselves. A successful politician must be in the majority or needed by the majority.
Therefore, politicians must cooperate and compromise with their fellow members. In America, this is logically accomplished by being a loyal and obedient member of either the Democratic or Republican parties.