The latest Times polling shows the extent of the challenge that President Biden faces and the strengths that Donald J. Trump retains. A yearning for change — as well as discontent over the economy and the war in Gaza among young, Black and Hispanic voters — may lie behind both. Nate Cohn, our chief political analyst, explains the surveys: New York Times/Siena College polls of Michigan, Wisconsin, Georgia, Nevada and Arizona, and the inaugural Times/Philadelphia Inquirer/Siena poll in Pennsylvania.