Congressional voting patterns

Party is the best single predictor of how a member of Congress votes. On most roll calls, Democrats vote with Democrats and Republicans with Republicans, and party line voting has grown stronger in recent decades as the parties have become more ideologically sorted. Party leaders, caucus pressure and shared ideology all push the same way.

District features and personal background matter at the edges, but none comes close to party as an explanation.

Each choice

• A. Population density does track party somewhat, since cities lean one way and rural areas the other, but party itself predicts votes far more closely.
• B. Members' economic backgrounds are similar across the parties, so wealth cannot explain why they split on votes.
• C. Most members are college educated whatever their party, so education level does little to separate their votes.
• D (correct). Party affiliation predicts roll call votes better than any other trait.
• E. Region shapes some votes, but members from the same region still split along party lines on most bills.