Electoral competition

[1] The Polity data series includes a measure of political competition.

[2] Political competitiveness can be affected by the proportionality between votes and seats, which can be represented by Gallagher index.

[3] In American federal elections, races for U.S. Senate tend to be more competitive than those for U.S. House of Representatives.

[4] Even in wave election years, the vast majority of U.S. House members keep their seats, with little pressure from the opposing party.

[5] Competition in U.S. House races has been in decline since at least the 1960s.

Political competition is defined as one minus the share of votes received by the largest party.
Political competition is one minus the share of the winning party's votes in a national election. It provides a measure of the winning party's dominance in the election.