This will be the Trump campaign agenda throughout 2020 if Mr. Biden gets the Democratic nomination — not to portray himself as ethical, but to get voters to say, “Well, both of them have scandals, so whatever.” And some political journalists will feel compelled to acknowledge the accusations against the Bidens. They’ll offer caveats, of course, as one news story from 2019 did, saying, “There’s no evidence that Hunter or his father acted improperly or violated any laws. But the arrangement, government ethics experts say, raises concerns.” The “raises concerns” part is the key — it will be just enough to plant seeds of doubt in voters’ minds about the Democrat’s ethical commitments.

But the Trump approach is nothing if not flexible. He’d use it against any Democratic nominee. If Mr. Sanders is nominated, Mr. Trump might pull from many things in the senator’s long political history. For one, Mr. Sanders honeymooned in the U.S.S.R. in 1988. It wouldn’t take too much effort for Mr. Trump to suggest that Mr. Sanders did something untoward or un-American during that 10-day trip, and he has friends in high places in the Russian government who could help in this regard.

If it’s Pete Buttigieg, he’ll accuse the former mayor of being racist through his leadership in South Bend, Ind., or of being corrupt through his consulting work with McKinsey. If it’s Elizabeth Warren, he’ll accuse her of racism, of fraudulently using ancestry claims to get into schools and of taking millions of dollars in shady legal consulting fees.

My argument here is not that Democrats should focus on picking a clean nominee who can’t be smeared with scandal. The leading Democrats are all pretty clean. Rather, I’m saying that Mr. Trump and his Republican allies will attempt to make the nominee look dirty, legitimately or not, no matter who it is. That’s his one go-to campaign tactic.

Now, it’s not obvious that this tactic works all that well. According to John Sides, Michael Tessler and Lynn Vavreck’s study of the 2016 election, “Identity Crisis,” media coverage of the general election was a net negative for Mr. Trump. Even while tearing down Mrs. Clinton’s reputation, he was still at a disadvantage.

On the other hand, the relentless focus on the email scandal most likely pressured James Comey, the F.B.I. director at the time, to announce the agency would review new material in the investigation of Mrs. Clinton in late October of 2016, and that may well have influenced the election’s outcome.

But once Mr. Trump starts going after the Democratic nominee this way, Democrats shouldn’t kick themselves for not picking a cleaner champion. No matter who it is, Mr. Trump will find a scandal.