Disunity is always a sin. The unity of the Church testifies to Her holiness. The unity of the Church is a sign of Her Orthodoxy, because this unity is a sign of its divine origin and freedom from the sinful human mixture. We see everything in the world as disuniting, even if it unites small entities. Politics gather many people together, and by doing this it gathers to disunite.
Political parties do the same.
Sectarianism is a greater sin because it does the reverse of what is exactly needed. All gatherings happening in the world are often related by some unreal links and driven by some opposite and competing goals. The associations, leagues, clubs, companies and same interest groups and gatherings come together for some interests that disunite not only one person from another but a whole society from another Religion often turns to become a disuniting factor among people, instead of being an oasis uniting man with his brother. In the Old Testament, the Samaritans in Palestine were disunited from the Jews. In this age, the Body of Christ is torn apart by religiosity among denominations.