Petrocik's theory of issue ownership states that parties have reputations for representing certain issues. In other words, each party owns certain issues. For example, Republicans own law and order, national defense and family values; Democrats own education, environment and civil rights. During elections, each party focuses on issues that it owns.
However, there are performance issues that are not owned by any political party. These issues include the economy and war. When the economy or a war goes well, this helps members of the president's party win races; when they underperform, members of the president's political party are less likely to be elected. Due to media exposure, it is irrelevant if Democrats rule Congress and a Republican is president.
Political Parties simplify vote choices by bundling issues. Elections create an incentive for parties to build a majority by: supporting issues that will attract a simple majority; unifying people with different interests.
Issue bundling creates coalitions of convenience. Coalitions of convenience consist of a variety of issues with no common ideological thread that are nonetheless attributed to a particular party. Rather than making decisions across a spectrum, voters need only choose between the Democrat Bundle and the Republican Bundle.
Voters often follow their party's coalition of convenience. For example, Republicans eventually agreed with Nixon when he supported free trade after previously opposing it.