State the policies of convergence
Explained here in the simplest case with assumption that there is full representation of voters; information is symmetric and single peaked preferences. But it is important to note that such policy convergence is generally not apparent on two grounds.
First, parties may not be so opportunist that they view to win the election anyhow. In fact, the image they recreate by diluting their policy stands affects their future as well. So they prefer for to implement the policy after they rescue the office. That is why we find few cases of complete convergence of policy stands, which, in other words, imply complete merger of two parties. Thus, the desire to win leads to a tendency towards convergence.
On the other hand, policy preferences may pull parties away from the median position. But despite this convergence processes, parties do not maximize the identical welfare function. Why? It may be because either party differs in perception of social welfare or they may propose which serves some special interest groups.