As Democrats engage in post-election self-flagellation, and pundits point to poor messaging and an inability to quell voter concerns about inflation to explain Kamala Harris’ loss, everyone seems to be ignoring how President Joe Biden’s biggest mistake by far cost the Democrats not just the presidency, but the House and Senate too.

Inaction at the border? The Afghanistan withdrawal? Not resigning the presidency?

All enormous errors, but not his worst. No, his worst error—and the thing that will forever stain what was otherwise an admirable presidency—was his failure to arrest former President Donald Trump on Jan. 21, 2021.

I am not arguing, as others have, that Biden should have arrested Trump because Trump was later able to resurrect his candidacy and succeed against the Democrats. Such an argument is morally flawed. In fact, it’s so morally flawed, it’s been pushed by Trump himself.

My argument, rather, is that by failing to arrest Trump immediately, Biden allowed the seriousness of Trump’s treasonous acts to diminish in the public’s collective memory, granting Trump’s supporters and others who might be swayed to his side the chance to believe that it was an open question as to whether or not he’d engaged in insurrection (when it definitely was not). It allowed the media, too, to make it seem like less than a fact, engaging in the usual horserace political coverage once it was election season, even though one of the candidates was a would-be usurper.

After the inauguration ceremony and issuing some quick executive orders on Day 1, Biden’s Day 2 should have begun with an order to the FBI to go and arrest Donald Trump.

Why didn’t the president do just that?

Simply put, he believed too much in the American people and our justice system. He took a hands-off approach, wrongfully calculating that the public would demand Trump be held to account for Jan. 6, and that the Republican members of Congress who had condemned Trump’s actions that day would come to their senses and refuse to ever support him again.

How’d that work out?

The problem with this approach—beyond putting far too much faith in the system and Merrick Garland—was that it meant simultaneously trying to portray Trump (accurately) as a threat to the nation with fascist tendencies, while allowing him to roam free and entertain guests at Mar-a-Lago. We arrested and charged many of those who’d been at the Capitol that day, yet failed to apprehend and prosecute the ringleader. Trump’s very freedom made people doubt his guilt, since humans have this terrible tendency to believe that we get what we deserve. If Trump had really engaged in a coup, they reckoned, why wasn’t he sitting in a federal brig?

No doubt taking such swift, decisive action would’ve come with a cost, including accusations that the arrest was politically motivated and possibly violent reactions from Trump’s base of support, which had already proven itself capable of such acts. And, of course, Biden already had a pandemic and its economic fallout to deal with.

Biden’s mistake is somewhat understandable. He wanted the nation to heal and felt that apprehending Trump would’ve damaged that process and distracted us from other important matters. But nothing was more important than taking a strong stand in favor of democracy. Biden should’ve acted quickly, then used an Oval Office address to explain his actions and bring the nation together. It would’ve sent a strong message at a time when the nation needed it. And, after what we all witnessed, it would’ve been met with tremendous, tremendous support. In fact, it would’ve likely put to bed forever the danger that is Trump, and possibly hammered the final nail into the coffin of Trumpism.

Instead, Biden let the wound fester, and let people forget what had happened that day—or at least lose perspective of what it truly meant—that Donald Trump had tried to overthrow an election, which means he’d tried to overthrow the government.

Biden allowed people to underestimate the danger Trump represents, and now that danger is all too real.