Could the plagiarism allegation that Joseph R. Biden Jr. faced in 1988 still be a problem? Mr. Biden dismissed complaints that he lifted passages for his speeches as “much ado about nothing,” but ended his presidential campaign six days later. What about “T-Bone,” a figure in Cory Booker’s stump speeches in 2007, who the senator from New Jersey was accused of making up? At the time Mr. Booker insisted T-Bone was “1,000 percent a real person,” but has never mentioned him again.

Or even the temporary restraining order issued last week against Michael Avenatti, the lawyer who was arrested on suspicion of domestic violence, and who has been talking himself up as a 2020 contender?…

“The rise of Donald Trump was a game changer. We are living in a stage of new normalcy,” said Mr. Avenatti, who rose to fame representing the pornographic actress Stephanie Clifford, better known as Stormy Daniels, who said she had an affair with Mr. Trump. Regarding the impact of the domestic violence allegation against him, which he has denied, Mr. Avenatti said, “I don’t think that any of it is disqualifying, but it depends on how it plays out.”