4.3 Political Party Systems

United Russia won 349 of the 450 Duma seats in September 2026. Nigeria's next presidential and National Assembly elections are set for 16 January 2027, so check whether the APC kept power. In Mexico, the PRD lost its national registration after the 2024 election.

• One-party system: one party controls the state and others are banned or have no real power.
• Dominant-party system: several parties compete, but one wins almost every time, often helped by state resources.
• Two-party system: two large parties alternate in power, even if others exist.
• Multiparty system: three or more parties have a real chance to win power or join coalitions.

Electoral rules: single-member district plurality pushes toward two big parties; PR allows more. The UK shows both sides of Duverger's law: Conservatives and Labour have dominated, but parties with concentrated support (like the SNP) or growing national support (like Reform UK) can still win seats.

Registration rules: Mexican parties lose their national registration if they win less than 3% of the vote, which is how the PRD lost its registration. Mexican parties may also form coalitions to back the same candidates. Nigeria's constitution bars ethnic or religious parties, and the electoral commission deregistered 74 parties in 2020.

Cleavages: regional and national divisions support parties like the SNP and Northern Ireland's parties.

Rules that protect a dominant party: in Russia only registered parties can run lists, and registration was made very hard for most of the 2000s. The list threshold rose to 7% for 2007 and 2011, election officials and courts have disqualified opposition candidates, opposition parties get little time on national TV, and governors' elections were scrapped from 2004 to 2012. In 2012 the membership needed to register a party fell to 500, which created many small parties that rarely threaten the dominant one.

Rules that ended dominance: Mexico's move away from one-party rule came with ending the dedazo, selling state firms that had fed patronage, giving states and cities more power, and creating an independent electoral institute.

A catch-all party tries to appeal to voters across classes and groups rather than to one ideology or group. The UK's two main parties have both moved this way at different times, and the PRI in Mexico long held together labor, farmers and business under one umbrella.