Will Joe Biden inevitably win the Democratic nomination for president? A month ago, many psephologists thought so, as national polls within two weeks of his April 25 announcement showed the former vice president with 41% of Democratic primary votes.

Four weeks later, that number has fallen to 33% — still formidable in a field of 24 candidates, but lacking the look of inevitability. His light schedule of campaign events suggests a lack of confidence in the stamina steadiness of a 76-year-old candidate, and his flip-flop on major issues strengthens that impression.