Why is party identification the single largest factor that determines how someone votes?

What will be an ideal response?

Partisanship is often thought of as a standing decision, meaning that voters continue to use it in their decision making unless convinced otherwise. It directly affects the vote because of the identification citizens have with their party. Party identification also indirectly affects the vote because it molds partisans’ views on political issues and candidate evaluations to fit with their partisan preferences. Therefore, voters view issues and evaluate candidates in ways that favor their party.