To say that a prosecutor cannot indict a sitting president is, by definition, to say that the prosecutor’s evidence must be given to Congress so that it may decide whether the president should remain in office. It means, in short, that should Mr. Mueller conclude he cannot indict a sitting president, he would also have to turn over all of the information he has uncovered to Congress.

A second problem for Mr. Trump is how Mr. Giuliani’s claims affect Mr. Trump’s possible refusal to testify before Mr. Mueller or a grand jury. If Mr. Giuliani is correct that Mr. Trump cannot be indicted, then the other idea being floated by Mr. Trump’s lawyers — that such testimony would amount to a “perjury trap” — makes little sense.

Perhaps the idea is that Mr. Trump still technically retains his right against self-incrimination because he could be prosecuted once he leaves office. But Mr. Giuliani’s claim highlights the absurdity of a refusal by Mr. Trump to testify: The president of the United States would be refusing to do what every other federal employee must do — provide evidence in a law enforcement proceeding — even when he faces no imminent criminal consequences.