Questions about former Vice President Joe Biden’s mental fitness are seeping into mainstream discussion, as Biden continues to make head-scratching remarks, and as his fellow Democratic candidates have started to broach the subject.

Former Housing and Urban Development (HUD) Secretary Julian Castro’s jab at Biden’s memory on Thursday night was widely interpreted as a veiled reference to Biden’s age and mental fitness — though Castro later claimed he wasn’t commenting on Biden’s age at all.

New Jersey Sen. Cory Booker similarly hinted at Biden’s mental fitness in a post-debate interview with CNN. “I think that we are at a tough point right now, because there’s a lot of people who are concerned about Joe Biden’s ability to carry the ball across the end line without fumbling,” Booker said.

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“And I think that Castro had some really legitimate concerns about — can he be someone in a long grueling campaign who can get the ball over the line? And I think [Castro] has every right to call that out,” he added.

Biden sparked questions about himself with a bizarre reference to record players while answering a question about the legacy of slavery.

“Social workers help parents deal with how to raise their children. It’s not that they don’t want to help, they don’t know what to play the radio, make sure the television — excuse me, make sure you have the record player on at night, the — make sure that kids hear words, a kid coming from a very poor school — a very poor background will hear 4 million words fewer spoken by the time we get there,” Biden said.

Zack Beauchamp, a writer with left-wing website Vox, argued that Biden’s strange answer “has to raise questions about Biden’s mental fitness for office, especially given his age.” (RELATED: Democratic Debate Gets Personal As Candidates Go For The Jugular)

In another odd moment Thursday night, Biden referred to Vermont Sen. Bernie Sanders as “the president.” Biden also referred to himself as the current vice president during a question about the Obama administration, saying: “I’m the vice president of the United States.”

The Daily Beast’s debate coverage noted that “over the course of three hours, and the post-debate spin as well, Biden’s mental standing was not just the subtext, but the focus of the conversation.”

Democratic Ohio Rep. Tim Ryan, who didn’t qualify for Thursday’s debate, was the first presidential candidate to raise questions about Biden’s mental acuity. Ryan said earlier in September that Biden is “declining” and lacks “clarity” when he speaks.

The Biden campaign didn’t return a request for comment for this article.

The 76-year-old former vice president has produced a steady stream of gaffes and mental slip-ups on the campaign trail.

In an appearance in Keene, New Hampshire, last month, Biden seemed to think he was in the neighboring state of Vermont. Biden slipped up again during a speech in Iowa last month, telling the audience that “poor kids are just as bright and just as talented as white kids.”

Biden has also confused the late former British Prime Minister Margaret Thatcher — who passed away more than six years ago — with both former British Prime Minister Theresa May and German Chancellor Angela Merkel.

In yet another error, Biden claimed he was vice president during the 2018 Parkland school shooting.

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