Former vice president Joe Biden at the Iowa State Fair on Aug. 8 in Des Moines. (Salwan Georges/The Washington Post)
Data analyst and political columnist

In Thursday night’s debate, Julián Castro tried out a potentially fruitful line of attack against poll leader Joe Biden. But he might have bungled the execution.

While sparring over the details of health-care policy, Castro repeatedly claimed that Biden had forgotten what he said only two minutes earlier. You can read the transcript here, but the subtext seemed clear: Castro was suggesting that Biden, in his old age, had become more forgetful and less mentally present.

In my view, Castro came off as overly aggressive. And I’m not sure that his attack will end up damaging Biden’s popularity or increasing his own. But broaching the issue of Biden’s age might have been a smart move in the medium term.

Gallup recently tested whether Americans would vote for a presidential candidate if they happened to belong to a specific demographic group — that is, if they’d vote for a qualified candidate from their party who happened to be gay or lesbian, a woman, black, an atheist, an evangelical, a Catholic or, yes, a septuagenarian. And many voters didn’t like older candidates.

Among all Americans, only 63 percent said they would vote for a candidate who is older than 70. That’s the third worst of any group Gallup tested — the only qualities that voters said made a candidate less desirable were atheism or socialism. Voters said they were more willing to vote for a Muslim, a candidate younger than 40, an evangelical Christian, or a gay or lesbian candidate than for an older candidate. Among Democrats, 65 percent said they would vote for a candidate older than 70, making septuagenarians the least popular group Gallup asked about.

Biden’s age — he’s 76 — is one of his weaker points in this primary. He seems to have built much of his candidacy on goodwill from the Obama era, perceived electability and relative moderation. He represents a bloc of the Democratic Party that is underrepresented on political Twitter and has (correctly or incorrectly) convinced many Democrats that he has the best chance of beating President Trump. But if Democrats believe that his age is a liability and start to question his electability, he might lose some support.

Being a little gray isn’t a deal breaker in a presidential primary, or even a general election. Trump is 73, and Hillary Clinton was in her late 60s during the 2016 campaign. Moreover, most Americans say they’re willing to vote for an older candidate.

But if Biden starts to exhibit some of the negative tropes of aging — forgetfulness, inability to stay on a topic, lack of physical endurance or mental toughness — some voters might decide to leave his camp. They might throw their support to a younger candidate, or one of the more spry septuagenarians. If wisdom comes with age, Democrats don’t have to choose between experience and vigor.

Watch:

Read more:

2020 Power Ranking: The winner of the third debate? Nobody.

Greg Sargent: Be very afraid. We’re missing something big about Biden’s age.

Alyssa Rosenberg: The Democratic debates are making me feel better about America

Helaine Olen: Why don’t voters care if a presidential candidate is old?

Evan Thomas: How old is too old to be president?

Jeffrey Sonnenfeld: Think Biden and Trump are too old for the White House? Take a look around.