The trip to Iowa was Mr. Booker’s fourth of this campaign, evidence of the need for a strong showing in this first caucus state for any hope of a successful run through to South Carolina, where Mr. Booker is hoping for a victory. The entrance of former Vice President Joseph R. Biden Jr. into the race puts added pressure on Mr. Booker, especially competing for black voters — a major piece of the Democratic constituency in the South, and a group that has largely supported Mr. Biden in early polls.

On the trail, Mr. Booker is still campaigning with an ebullient style, recording selfie videos for voters and cracking self-deprecating “dad jokes” at every stop. He even broke briefly into song during an interview, singing a few bars of “My City of Ruins” by Bruce Springsteen.

But stuck in the low single digits in both national and early state polls, with a middling small-dollar fund-raising operation, he has started to sprinkle his unity-themed stump speech with some slight contrasts to other Democratic candidates.

“I’m one of those Democrats to say we need to start doing things that make people’s lives better, because Democrats have done things that have made people’s lives worse,” Mr. Booker said at a house party in Newton, referring to the 1994 crime bill that Mr. Biden supported while he was in the Senate, and which experts say contributed to mass incarceration.