Back in 2020, Joe Biden stood out in a crowded Democratic primary field filled with younger, more historic candidates. Voters worried that Mr. Biden was too moderate, too uninspiring and too old.

One of his challengers, former Housing and Urban Development Secretary Julián Castro, got a lot of attention for his willingness to echo those concerns.

But after going hard at Mr. Biden in a debate, his campaign fizzled and Mr. Castro, once a rising star in the Democratic Party, left Washington altogether. To some, it seemed like evidence of the consequences of stepping out of line with the party.

Heading into the 2024 election, as voters grapple with the same questions about the incumbent president, Astead sits down with Mr. Castro to explore the party’s code of silence surrounding Mr. Biden’s primary alternatives and his advanced age.

For more information on today's episode, visit nytimes.com/therunup.