WASHINGTON (AP) — Americans' habit of sorting themselves into Republican and Democratic regions is rewarding GOP Senate candidates because of their success in small conservative states.

Republican senators are entrenching themselves in small states that elected Democratic senators until recently. These states solidly back GOP presidential nominees.

States are increasingly likely to vote for the same party in Senate and presidential races.

Emory University political scientist Alan Abramowitz says Senate Republicans benefit because small states tend to be more conservative.

Each state has two senators. Therefore a party can prosper by winning Senate races in numerous small states even if most Americans don't back the party.

President Barack Obama carried at least 11 of the 15 most populous states in both his elections. He lost 20 of the remaining 35 states.