Joe Biden came into Tuesday’s primaries riding a wave of voter confidence about his chances of victory in November. Democratic primary voters in Missouri and Mississippi are more likely to think the former vice president could defeat President Donald Trump in the general election, compared with Biden’s top rival, Vermont Sen. Bernie Sanders, according to AP VoteCast surveys in the two states. That perception of electability is part of what is drawing Democrats to rally around the more moderate Biden, who has emerged as the Democratic primary front-runner thanks to moderate and conservative voters moving quickly in his direction. The...