FORMER NEW YORK mayor Rudolph W. Giuliani does not believe that President Trump should sit down with special counsel Robert S. Mueller III and simply tell the truth. “Every lawyer in America thinks he would be a fool to testify,” Mr. Giuliani insisted to ABC News’s George Stephanopoulos on Sunday. Mr. Giuliani, now working as one of Mr. Trump’s lawyers, also would not rule out the possibility that, even if Mr. Trump appeared, he would invoke the Fifth Amendment and refuse to answer questions.

Mr. Giuliani may be right as a lawyer (though he hasn’t exactly inspired confidence with his legal advice over the past week), but he has it wrong as a citizen.

The Fifth Amendment indeed offers an essential right against self-incrimination to the ordinary client. But the issue is not what should be expected from an average client, or even a famous one. It is what the public should expect of the president of the United States in a probe involving serious questions of national interest. Previous presidents answered investigators’ questions in these circumstances out of respect for law enforcement, a sense of duty and a calculation that refusing to cooperate with a bona fide federal investigation would carry a political price. The same expectations should apply to Mr. Trump.

Why should Mr. Trump fear testifying, if he is prepared to testify honestly? Mr. Giuliani claims to worry that former FBI director James B. Comey will manipulate the special counsel and the courts into believing that truthful testimony from Mr. Trump is perjurious. Leave aside the fact that, whatever else one thinks about his actions, Mr. Comey has given no reason to believe he has been dishonest. In the same Sunday interview, Mr. Giuliani effusively praised a federal judge who criticized the special counsel last week. So did Mr. Trump on Monday, tweeting that “there is a Court System in place that actually protects people from injustice.” If Mr. Trump is right — this time — about the court system, he has nothing to fear from appearing before a duly convened grand jury. In fact, what Mr. Giuliani and Mr. Trump are really arguing is that the criminal-justice system has integrity when things seem to be going their way and is corrupt when they do not like the outcome.

This is a toxic view for the president to maintain, turning matters of fact and law into another tribal battle of my side vs. the other side. The questions the Trump team expects Mr. Mueller to ask became public last week; they deserve answers, and not just before the special counsel. What contact did the president have with Russian officials in a 2013 trip to Moscow? Did the president know anything about any outreach to Russia by campaign manager Paul Manafort or others? What was the decision-making behind firing national security adviser Michael Flynn?

No one should accept secrecy and obfuscation in the face of such vital questions. Every lawyer in America should know better.

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