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A Democrat facing questions about alleged buckraking abroad. The specter of possible foreign interference. And a Republican standard-bearer eager to use any argument or insinuation to tag his opponent as part of a corrupted system.

I spent much of this past weekend in Iowa, watching the 2020 primary field and suffering from a serious case of 2016 déjà vu.

Three years ago, Donald J. Trump cast Hillary Clinton as a symbol of Washington self-dealing and corruption, attacking her family foundation for raising money abroad and her use of a private email server.

Now, the president is trying to cast Joe Biden in the role of Mrs. Clinton.

Over the weekend, Mr. Trump’s team launched a full-scale assault on Mr. Biden, with top cabinet officials taking to the Sunday talk shows to push for greater scrutiny of Mr. Biden and his son, Hunter, who did business in Ukraine while his father was vice president.