SINCE THE Ukraine scandal erupted, Rudolph W. Giuliani has been ubiquitous on cable television and has responded almost daily to questions from reporters, sometimes with emoji-laden texts. But the man who describes himself as President Trump’s personal lawyer is defying a congressional subpoena for documents and says he will refuse all cooperation with a House impeachment investigation.

Mr. Giuliani appears to have no legal grounds for his contempt of Congress. But it’s understandable why he might be reluctant to answer investigators’ questions. The evidence is mounting that the former New York mayor hijacked U.S. foreign policy in Ukraine to serve the interests of his clients — and that he attempted to do the same with U.S. policy toward Turkey.

According to the testimony of White House and State Department officials, Mr. Giuliani upended U.S. diplomacy in Ukraine by feeding Mr. Trump false stories and conspiracy theories about the 2016 election, Joe Biden and the U.S. ambassador in Ukraine. When Mr. Trump ordered the ambassador home in May, the winners included two of Mr. Giuliani’s clients, Lev Parnas and Igor Fruman.

Mr. Parnas paid Mr. Giuliani $500,000 for consulting work, purportedly for a security company Mr. Parnas founded. It’s not known where the money came from; Mr. Parnas and Mr. Fruman had had problems with unpaid debts. But according to federal prosecutors, they received two wire transfers of $500,000 from an unidentified Russian businessman in September and October 2018, just after they hired Mr.  Giuliani.

Mr. Giuliani told Reuters the money he was paid did not come from a foreign source. But he wouldn’t say where it came from. That would be a logical target for investigation by the federal prosecutors in Manhattan who, according to the New York Times, are investigating whether Mr. Giuliani broke a law requiring lobbyists to disclose work done on behalf of foreign interests.

Then there is the case of Turkey, which, according to reporting by The Post, was the subject of heavy lobbying by Mr. Giuliani of Mr. Trump in 2017. His attempt to persuade Mr. Trump to release a Turkish gold trader charged by federal prosecutors with conspiring to violate sanctions against Iran reportedly troubled then-Secretary of State Rex Tillerson, who thought the proposed intervention in a criminal proceeding was improper.

Meanwhile, Mr. Giuliani was trying to persuade Mr. Trump to extradite an exiled Turkish cleric, Fethullah Gulen, back to his home country — a step that U.S. officials strongly opposed but that the president appeared open to. Both the extradition and the release of the trader were top priorities of Turkish President Recep Tayyip Erdogan, whom Mr. Giuliani traveled to Turkey to consult with in February 2017.

At the time, Mr. Giuliani was on the legal team of the gold trader, Reza Zarrab, who eventually pleaded guilty. That, Mr. Giuliani told The Post, meant he need not register as a foreign agent. Yet his activity resembles that of former national security adviser Michael Flynn, who belatedly registered after lobbying for the extradition of Mr. Gulen while being paid by a Turkish company with ties to the government.

Federal prosecutors will determine whether there are grounds to believe Mr. Giuliani violated the law. What’s already clear is that he perverted U.S. foreign policy for his private ends, and those of Mr. Trump. That ought to be reason enough for Congress — and, if necessary, the courts — to compel his cooperation in the impeachment investigation.

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