Why does Washington warn Americans against "the spirit of parties"?

He thinks the spirit of parties are the worst because, "It exists under different shapes in all governments, more or less stifled, controlled, or repressed; but, in those of the popular form, it is seen in its greatest rankness, and is truly their worst enemy." A government with great power is prone to abuse it, a government needs to distribute the powers so no part of the government is strong enough to over power the other.