A two-party system exists when two political parties dominate political discourse and election campaigns. In such a system, the chief executive and (on average) seventy-five percent or more of the members of the legislature are members of one "major party" or the other. The rest might come from "minor" parties (not members of the two-party duo) or hold their offices with no affiliation.
"Third party movements" frequently arise in the politics of a country having a two-party system. Typically such movements are as weak as they are frequent. Some issue arises that neither major party wants to address. But one of two things will then happen, often in the next election cycle:
- One or the other major party will co-opt the issue, or
- Both major parties will collude, openly or by default, to marginalize this issue.
When this does not happen, and especially when one major party abruptly loses electoral strength, a minor party might replace a major party. The present Republican Party replaced the Whig Party in the 1850s over the issue of slavery, for instance.
Sometimes a two-party systems seems to have greater endurance than one might expect. Attempts to marginalize a "third-party issue" do not succeed in making people stop talking about it. This leads to charges, like that which George C. Wallace raised in the 1968 Presidential Election, that
|“||There's not a dime's worth of difference between the two parties.||”|
A two-party system can result in a constitutional republic where there is a strict separation of powers between the branches of national government, as in the United States.