Go ahead. Underestimate him.

In a Democratic presidential debate in September 2019, Julian Castro thought he heard Joe Biden say something that contradicted himself, and he pounced on the opportunity to suggest that Biden was over the hill. "Are you forgetting what you said two minutes ago?" he demanded. "Are you forgetting already what you said just two minutes ago?"

Propelled by this moment of triumph, Castro went on to become a member of the board of directors of a Washington think tank.

As it turned out, it was Castro who was confused about what Biden had said. If the 2020 campaign proved anything, it's that underestimating Biden is dangerous. But Republicans persist in depicting him as a decrepit specimen who is wholly inadequate to his presidential responsibilities.

Sen. John Cornyn, R-Texas, expressed concern last month that the president was not doing cable news interviews or tweeting much. "Is he really in charge?" he tweeted. When Biden addressed Congress, Fox News host Tucker Carlson claimed to hear "a 78-year-old man losing his grip." Wall Street Journal columnist Holman Jenkins Jr. wondered if Biden is "a man of diminished capacities" who is "making himself a prop for an agenda that he may not quite grasp."

This sounds eerily like what Biden's detractors said about him during the campaign. First it was from the left, with supporters of Bernie Sanders putting out talking points insisting that Biden was in "obvious cognitive decline." Sen. Cory Booker said, "There are definitely moments where you listen to Joe Biden and you just wonder."

Biden somehow stumbled his way to the nomination, vanquishing a huge field of younger and supposedly sharper rivals (and an older one, Sanders). But that didn't stop Republicans from insisting that he was conducting a mostly virtual campaign — "hiding in the basement" — not because of the pandemic but because he was too addled to appear in public.

Then-President Donald Trump predicted that if Biden should somehow win, "They are going to put him in a home, and other people are going to be running the country." An editorial in the Wall Street Journal warned that Biden might "duck the debates" because "his handlers are trying to protect him from doubts about his cognitive capacity."

For a dementia victim, he did amazingly well. In fact, polls indicated that voters thought Biden got the best of Trump in all three faceoffs. He also won the election, over an incumbent president who called him "the worst candidate in the history of politics."

But critics continue harping on this losing theme. In March, Fox News contributor and The Hill columnist Joe Concha demanded to know why Biden hadn't held a news conference or given a speech before Congress. Biden has since done both, and handled both with competence and aplomb.

His foes still imagine that they can make people accept something that is plainly untrue. But Americans prefer to believe what they see with their own eyes. Biden's approval rating is higher than Trump's ever was, and an ABC News/Ipsos poll released Sunday found that 64% of Americans are optimistic about the direction of the country.

The portrayal of Biden as disconnected from reality is particularly creative coming from people who shrugged off Trump's fantastical claims, nonstop lies, strange mispronunciations and unhinged rants. They had no problem with a Republican president who spent an outlandish amount of his time watching TV and fulminating on Twitter while neglecting the more important duties of his office.

The image of Biden as helpless is hard to reconcile with the parallel claim that he is ruthlessly transforming America into a woke Marxist dystopia. But conservatives square this circle by theorizing that Vice President Kamala Harris is actually running the show. Their paradoxical accusation: Biden is hiding to conceal the fact that he's not in charge, while Harris is hiding to conceal the fact that she is.