Which of the following statements about the political opinions of Democrats and Republicans in the public is the most accurate?

Membership in either the Democratic or Republican parties is one of the most important factors affecting political attitudes.

Differences between the political opinions of Democrats and Republicans are smaller today than during any other period in the last $40$ years.

Republicans and Democrats have similar opinions on all political issues with the exception of national security.

The differences between the political opinions of Democrats and Republicans have been the same throughout American history and have not become more intense over the last $40$ years.