After a year of stages crowded with flash-in-the-pan phenoms and forgettable faces, the race for the 2020 Democratic presidential nomination is about to yield a totally new kind of debate: Joe Biden versus Bernie Sanders, mano a mano.

That’s bad news for Biden, and his team knows it. We know they know it because they won’t accept Sanders’s offer for a one-on-one debate about health care policy; because they’re lobbying to make the debate format less demanding; because Biden is refusing invitations for extended interviews on cable news; and because he was recently spotted handing out leaflets to reporters after an event instead of talking to them directly — clearly a workaround designed by his staff to prevent him from putting his foot in his mouth.

His staff are right to be worried. The elder statesman is prone to lapses in coherence, to put it politely. The signs have been there for months: when Biden asked supporters to “go to Joe 30330 and help me in this fight” in the August debate, the Guardian called it “one of the most cryptic gaffes of his career.” But it was only the beginning.

In his bid for the nomination, Biden has forgotten Barack Obama’s name repeatedly, proclaimed he was running for Senate, declared that over 150 million people have been killed by gun violence since 2007, confused his wife with his sister, confused Angela Merkel with Margaret Thatcher, and confused Theresa May twice with Margaret Thatcher. He’s issued an endless litany of baffling and embarrassing statements, including, “Tomorrow’s Super Thursday,” “We choose truth over facts,” “Poor kids are just as bright and just as talented as white kids,” “Why, why, why, why, why, why, why?” and “We hold these truths to be self-evident: all men and women created by the, go, you know the, you know the thing.”

Biden’s allies have attempted to explain all this away by pointing out that Biden has long struggled with a stutter. It’s true that Biden stuttered as a child, but it is not true that he’s always labored to verbally convey his ideas. For decades Biden’s speech was lucid, convincing, at times even charismatic. In 2012, he was lavished with praise for verbally eviscerating Paul Ryan during a vice presidential debate. Less than ten years ago he was confident, composed, funny, and alert when he spoke in public. Nowadays, his campaign makes a concerted effort to limit his exposure to prevent the likelihood of verbal slip-ups, confessing that they tend to happen “late in the day.”

If Biden’s stutter has anything to do with his constant verbal fumbling, even his allies have to concede that it somehow seemed under control for decades and has now reemerged. So, what’s triggering its comeback? The answer to many — including many in the Democratic Party before they decided to consolidate around him — is some kind of mental decline caused by aging. Some have speculated dementia specifically, but there’s no need to attempt a diagnosis. Many neurological conditions become more probable and pronounced with age. Some kind of age-related cognitive impairment seems more likely to underlie the issue than stuttering, given that not all of Biden’s gaffes involve searching for the right word and failing to find it.

For example, Biden is also given to sudden displays of aggression, such as when he called a voter fat and challenged him to a physical contest, and extended incoherent ramblings, such as when he answered a question about the legacy of slavery by complaining that parents are failing to make their children listen to record players at home. And when he tells blatant untruths, which is often, he seems less like someone who’s being intentionally deceitful than someone who can’t remember the truth — for example when he said his son had acted as Attorney General or that he was arrested for protesting apartheid in South Africa.

Whatever its root cause, Biden’s extreme incoherence poses a serious liability in a contest with Donald Trump. The potential insensitivity of pointing it out is eclipsed by the danger of nominating someone who can’t hold his own in a general election. Trump’s mockery of Biden has already started, and offers a preview of what’s to come. Biden “makes a mistake every time he speaks,” says Trump. “I can just see these handlers … ‘Alright, get him off now, he’s been up there long enough!’ So they’re screaming, ‘Get off! Get off! Sleepy Joe, get off the stage! Please! … Damn it, he should have left sooner.’”

Over the weekend, Biden appeared for a mere seven minutes in St Louis, where he said, “We cannot get reelect, we cannot win this reelection, excuse me, we can only reelect Donald Trump, if in fact we get engaged in this circular firing squad here,” a bungled attempt to castigate his opponent Bernie Sanders for criticizing Biden’s efforts to cut Social Security. Trump ridiculed Biden by posting a clip of the speech that was edited down to say, “We can only reelect Donald Trump.”

News outlets sympathetic to Biden described the clip as manipulated, fake, or doctored. The editing was indeed dishonest, but while the full clip may clarify their meaning, they also show Biden struggling to articulate himself in public on one of the very recent few occasions that he’s ventured to do so.

Biden’s entire candidacy is premised on the idea that he’s the safe option. He stands for little, and even those close to him appear unimpressed by him. But at least nominating him is less perilous than nominating the radical Bernie Sanders, right?

Many of us on the Left argue that this is an incorrect assessment for political reasons. Biden’s record contains ugly betrayals of huge swaths of the electorate, from championing NAFTA, to authoring the 1994 crime bill, to supporting the Hyde Amendment, to selling Democrats on the Iraq War, to muscling through the 2005 bankruptcy bill that intensified the devastation of the economic recession. Between this past record and his forward-looking promise that “Nothing would fundamentally change,” his centrist politics alone make him an unappealing and therefore unsafe nominee.

But even if you find the political case unconvincing, it’s incumbent on you to at least give his mental state some serious consideration. If Biden is nominated, he’ll be forced into the spotlight every day for four months, without reprieve and with nowhere to hide. You can bet he’ll say many more things he will regret. He’ll no doubt make more gaffes than any candidate in history, and Trump will make the most of his windfall.

Running someone experiencing rapid cognitive decline against a mean-spirited bully who relishes the exploitation of weakness is worse than a risk. It’s a guaranteed disaster.