A single-issue party is a political party that argues and promotes legislation for a single issue. Green parties in various nations, for example, started off as parties focusing solely on the environment.
Single-issue parties are rare in the United States. This is largely due to the two-party system that dominates. Historically, third parties either lose power in a short period of time or replace another party. Single-issue interest groups, such as the National Rifle Association, yield significant influence over political parties, and their support can provide a significant boost to a candidate or entire party.
In countries with parliamentary systems, single-issue parties are more influential. Because power is divided into coalitions of multiple parties, voters are more likely to vote for single-party candidates. No elected politician is able to focus exclusively on a single issue, however. Politicians must vote on a broad range of issues, and single-issue party politicians generally vote with the parties with which they are aligned.
Over time, single-issue parties often adopt a wider range of issues to target. Green parties, for example, tend to focus on liberal and social causes when their power increases. Likewise, parties focusing on gun rights are likely to focus on conservative or libertarian ideals over time.