President Trump now knows who his general election opponent will be and, to hear his campaign and its allies tell it, it’s a Democrat who is just not that with it.

The Trump War Room video mashups are dominated by montages of former Vice President Joe Biden, the presumptive Democratic nominee, stumbling and fumbling his way through speeches and debate responses. Interspersed throughout the greatest hits — “Make sure you have the record player on at night,” odd references to primary opponents Bernie Sanders and Cory Booker as “president,” failing to correctly identify his current location, such as when he mistook New Hampshire for Vermont — are Democrats and liberal pundits expressing concern on camera. One of the videos, “You Just Wonder,” takes its title from a Booker quote: “There are definitely moments where you listen to Joe Biden and you just wonder.”

Trump hasn’t hesitated to play on these fears."I don't know why President Obama hasn't supported Joe Biden a long time ago. There's something he feels is wrong," Trump said on Wednesday, musing about his predecessor’s neutrality during the competitive phase of the Democratic primaries on the day Sanders suspended his campaign. "He knows something that you don't know, that I think I know.” Some Trump surrogates and media supporters have gone so far as to suggest Biden is suffering from dementia.

Long gaffe-prone, at 77, Biden’s frequent verbal slips, now believed by some to be worse than ever, are cited to raise questions about his mental acuity and whether he still has the stamina for the rigors of a campaign, much less the presidency. If he wins in November, weeks away from his 78th birthday, he will be older entering the White House than Ronald Reagan was leaving it. Before Trump, Reagan was the oldest man ever elected president. “As a smart friend said last night, 'Joe Biden has spent his entire life trying to succeed in presidential politics,'” quipped Fox News’ Tucker Carlson, “'and now he has. Too bad he’s not there to enjoy it.'”

Yet it is a risky strategy for Trump, who at 72 is no spring chicken himself and whose attacks on Biden could be seen as mean-spirited. “I do worry about Joe,” confessed one Democratic consultant. “But Trump running on this is like the pot calling the kettle black.” At the same time, the country is in the midst of a pandemic, and these issues could at the very least neutralize criticisms of Trump’s own age and verbal miscues. “I am a young, vibrant man,” the president boasted last year. “I look at Joe, I don’t know about him.”

“Everybody likes Joe,” said Republican strategist John Feehery. “The question is: Is he mentally vibrant enough at his advanced age to lead the country? It’s a valid question given his uneven responses to this crisis.”

Trump argues that Biden will be a doddering figurehead while younger Bernie Bros will really be in charge. “They are going to put him in a home and other people are going to be running the country,” Trump said at a North Carolina rally. Speaking at the Conservative Political Action Conference, Trump said it didn’t matter that Biden was less left-wing than Sanders: “Joe is not going to be running the government, he’s just going to be sitting in a home someplace and people are going to be running it for him, and they will be radical-left socialists.”

“Frankly, it would be criminal if Trump didn’t go after Biden’s mental state, particularly given the recent reams of video footage that suggest the former vice president might not be all there,” said Republican strategist Ford O’Connell. “Often in interviews and on the stump, Biden has shown signs of forgetfulness and of cognitive decline. There is no question that he has lost a few steps since being vice president. Let’s remember, Biden wants to be president of the United States, one of the most demanding jobs in the entire world.”

Already, “Sleepy Joe” has entered the pantheon of Trump nicknames alongside Crooked Hillary, Low Energy Jeb, Lyin’ Ted Cruz, and Little Marco Rubio. These epithets helped define Trump’s opponents, and operatives close to the president’s campaign say the focus on Biden’s vitality is no accident. “With Hillary, the Trump campaign was throwing everything at the proverbial wall to see what would stick,” said one. “Trump world has been kicking this idea around since the early days of his reelection campaign. So their strategy on this subject is well thought out.”