Donald Trump and Joe Biden come into Thursday’s presidential debate as incredibly well-known quantities defined by shared unpopularity and competing weaknesses. But their most important liabilities — for the incumbent, his decrepitude and his record on inflation; for the challenger, an unfitness distilled and confirmed by the events of Jan. 6 — feel too well known to be worth discussing further until we see what happens on the stage.

Maybe Biden’s entire campaign will implode when he answers a question about inflation with a Grandpa Simpson ramble about the price of a frozen custard on Rehoboth Beach in 1968.

Maybe Trump will turn into Colonel Jessep under Jake Tapper’s questioning and claim full responsibility for the 2021 riot at the Capitol.

But before the two men meet with those or other destinies, it’s worth giving some space to their crucial secondary weaknesses, the places where each candidate’s support might be undermined among those “double-hater” voters who regard each candidate’s primary liabilities as canceling the other’s out.

For Biden, that weakness is foreign policy, the deteriorating condition of world order since he took the oath of office. There’s been an endless debate about whether voter nostalgia for the Trump-era economy is justified, or whether it lets Trump off the hook for the Covid-driven economic crisis of 2020.

But nostalgia for the Trump-era geopolitical landscape seems entirely reasonable: Before his defeat there was no Russian invasion of Ukraine, no brutal struggle in the Holy Land, and a weaker alignment of anti-American powers rather than the increasing consolidation by our rivals in Moscow and Beijing and Tehran and even Pyongyang.

The Biden administration would obviously deny responsibility for these deteriorations, and argue that Biden has managed a set of extremely difficult situations better than a dictator-sympathizing Trump would have done. And as a defender of the Afghanistan withdrawal despite its disastrous implementation, and a partial defender of our attempted balancing of risks and evils in Ukraine, I have some sympathy for the White House’s self-justifications.

But I also think that liberal investment in the idea of Biden as the great defender of the liberal international order against isolationism and reaction has made it hard for Democrats to reckon with how much more stability the American-led order seemed to enjoy under the crudely transactional machtpolitik of Trump.

That contrast means that if there is a positive case for Biden’s foreign policy, it can’t just be made with rote warnings about how Trump will unravel NATO and let our enemies run roughshod — not when those enemies seem so much more aggressive under Biden than before.

Trump’s crucial secondary weakness, meanwhile, isn’t a matter of his record but his promises: specifically, his commitment to a sweeping 10 percent tariff on imported goods as the big-ticket economic policy of his potential second term.

When he first ran for president in 2016, Trump’s protectionism was part of a general pivot away from Tea Party libertarianism and the Mitt Romney-Paul Ryan austerity agenda, in a landscape where America had a lot of fiscal space to work with and it was possible to promise all kinds of sweeteners to balance out the cost of trade wars.

But in the current environment, a Trumpian promise not to cut Medicare and Social Security feels like old news, an expectation from his supporters. And there’s not much room for credibly promising other giveaways, whether new tax cuts or new spending, when inflation is still relatively high.

That throws the immediate costs of a 10 percent tariff, which might fall heavily on an inflation-weary middle class, into particularly stark relief. Some of those costs could be potentially offset, especially if the more serious supporters of a tariff policy were designing it. But that’s a complicated argument to make, and complicated policy arguments are not a Trumpian forte. So the tariff pledge presents a political target much like John McCain’s promise to abolish the tax exemption for employer-provided health insurance in 2008, which Barack Obama successfully portrayed as a tax increase on the middle class.

Especially when you combine the 10 percent tariff with Trump’s promise to keep his first term’s big corporate tax cut in place or even deepen it: Higher middle class prices and lower corporate tax rates are ideal grist for what has long been the most effective Democratic campaign against Republicans — the kind that portrays the G.O.P. as robbing the middle class for the benefit of plutocrats.

Can Biden prosecute that case cogently and in a way that doesn’t just remind voters of his own inflationary policies? I’m doubtful. But then you might also doubt that Trump can make a case against Biden’s struggle to contain America’s authoritarian enemies that doesn’t remind voters of his own authoritarian impulses.

And with both questions, both doubts, we’re back to each candidate’s primary weaknesses, in whose shadow — on Thursday night and thereafter — all secondary debates will inevitably take place.