Former vice president Joe Biden may not have clinched the nomination just yet, but he effectively became the Democratic nominee Tuesday night as he crushed Sen. Bernie Sanders (I-Vt.) in the Michigan, Missouri and Mississippi primaries. Trailing in the polls and with a majority disapproving of his job thus far, President Trump starts behind. Trump can change that only if he tackles Biden’s biggest weakness: himself.

It’s no secret that Biden has lost a few steps as he has aged. Time and again, he makes glaring errors when facing unscripted moments. He told South Carolina supporters he was running for the Senate. He also forgot the most famous line from the Declaration of Independence. And that’s just in the past few weeks.

Biden’s frailty doesn’t stop with his mind. He visibly tires during debates after the first-hour mark. In fact, the Sanders campaign recently accused the Biden camp of pressing for a sit-down format debate, as opposed to a typical two-hour standing debate, for Sunday’s one-on-one appearance with Sanders, which the Biden campaign denied.

Trump is not one to pull punches against his adversaries — in fact, he and his campaign have already begun targeting Biden. He will surely ratchet up the attacks on Biden soon and double down on his charges that Hunter Biden’s involvement with Burisma and a Chinese investment firm shows the Biden family is corrupt. But those attacks will, without more evidence, preach only to the converted. To win new supporters, he must raise doubts about Biden’s fundamental fitness for the job. And that means getting personal.

The president should first make the campaign a test of endurance. Fears of the coronavirus will limit large rallies for the foreseeable future, but these fears could lift by the summer. (If they haven’t by then, Trump is probably a goner anyway.) Once that happens, Trump should hold as many public events as possible in as many states as he can schedule, including exposing himself to frequent questions from average voters and local media. The contrast with Biden would be stark and noticeable.

Trump should also taunt Biden should he fail to pick up the pace. He can rightly say that the presidency is a stressful job that requires constant unscripted interactions, lots of travel and frequent decisions. If Biden can’t show he’s up to the demands of a campaign, why should voters think he can do the job once elected? “A Tough Man for Tough Times” would not be a bad slogan to contrast with Biden.

Every political strategist knows that Trump doesn’t have to win the popular vote; he just needs to lose it by a small enough margin that he can win the electoral college. If Biden’s lead narrows from its current 6.5 points to closer to 3, Trump has at least a 50-50 shot at prevailing.

That would mean Biden would have to take the risks that come from regular, active campaigning, just as Biden’s inability to hold his early lead in the primaries forced him to abandon his initial attempt to limit public appearances.

This approach could backfire on Trump if focuses on the wrong Biden. The former vice president gets emotional when defending his family, and he could gain public sympathy if Trump goes after Hunter too hard. Better to focus on Biden’s own stamina and competence, qualities many voters already have doubts about and qualms an emotional outburst cannot set to rest.

Many will say this is cruel, but the campaign against Trump has been cruel since day one. He is regularly labeled a racist, a traitor, a fascist and more. Trump would rightly say he’s not just fighting fire with fire; he’s focusing on the campaign’s core question: Who is fit to do the job? It turns his own weakness into a potential relative strength. You don’t have to like Trump to knows he’s always in charge of his administration. Biden’s weakness raises legitimate doubts about whether he’ll be merely a figurehead if elected.

Democrats have made the least worst choice by picking Biden over Sanders. Don’t be surprised if Trump makes them rue that decision sooner rather than later.

Use the Post Opinions Simulator to pick a state and see what might happen in upcoming primaries and caucuses.

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