Median voter. A two-party convergence around the median voter's position is an electoral equilibrium outcome. In two-party competition on a single dimension, electorally motivated parties have incentives to adopt a policy around the median voter's preference, approaching each other and converging in their policy or ideological positions. Once they converge, neither party has electoral incentives to move away from the other party.
Incumbent advantage. Electoral competition is asymmetric between the government and the opposition. The incumbent party in government can gain advantage in electoral competition by providing or hiding information on its record to obtain credibility.