A PILLAR of the Republican defense of President Trump during the impeachment inquiry has been that he had good reason to believe Ukraine intervened against him during the 2016 presidential election — and he therefore was justified in demanding that the new Ukrainian government investigate. That made the testimony Thursday of Fiona Hill, who served Mr. Trump as the National Security Council’s senior director for Europe and Russia for more than two years, particularly important. Ms. Hill, one of the nation’s top authorities on the regime of Russian President Vladi­mir Putin, warned that the Ukraine intervention narrative was not only “fictional” but also potentially damaging to U.S. national security.

U.S. intelligence agencies, the Senate Intelligence Committee and former special counsel Robert S. Mueller III have all found that Russia meddled in the 2016 election in support of Mr. Trump. But the president and some of his more fanatical followers, such as Rep. Devin Nunes (R-Calif.), have clung to the idea that the intervention came from Ukraine, and that it was aimed against Mr. Trump. Mr. Putin himself has echoed the Republican arguments. “Thank God no one is accusing us of interfering in the U.S. elections anymore,” he said Wednesday. “Now they’re accusing Ukraine.”

This false narrative, Ms. Hill bluntly declared, “has been perpetrated and propagated by the Russian security services themselves.” It is dangerous not only because it clouds the truth of what happened in 2016. It has also had the effect of weakening and politicizing U.S. support for Ukraine, which Russia still seeks to dominate. It facilitates what Ms. Hill said is Russia’s intention to intervene in the 2020 election.

“Right now, Russia’s security services and their proxies have geared up to repeat their interference,” Ms. Hill said. “We are running out of time to stop them. In the course of this investigation, I would ask that you please not promote politically driven falsehoods that so clearly advance Russian interests.”

Unfortunately, Mr. Nunes paid no heed. He raised, as he has throughout the hearings, an eccentric mélange of conspiracy theories and false charges about Ukraine. Among them are the claim that a Ukrainian journalist who reported on corrupt payments by a Ukrainian political party to Mr. Trump’s campaign manager was trying to damage Mr. Trump, something the journalist has called a “lie”; and the false allegation that a D.C. political consultant coordinated collaboration between Ukrainian officials and the Clinton campaign.

The most exotic theory of all was referenced by Mr. Trump in his July 25 phone call with Ukrainian President Volodymyr Zelensky: that Ukraine rather than Russia hacked the Democratic National Committee and that the server has been secreted in Ukraine. As Ms. Hill testified, Mr. Trump was told by some of his top aides that the theory was false. Nevertheless, the president insisted that Mr. Zelensky announce an investigation of the charge.

Both Ms. Hill and David Holmes, a political counselor at the U.S. Embassy in Kyiv, joined the host of officials who have now testified that the 2016 election investigation and another of Joe Biden were conditions for Ukraine obtaining a White House meeting that Mr. Zelensky sought. Mr. Holmes said he concluded that military aid was held up for the same reason. They both referenced former national security adviser John Bolton, who described the exchange of investigations for an Oval Office meeting to Ms. Hill as a “drug deal” and who told Mr. Holmes the release of military aid depended on whether Mr. Zelensky could “favorably impress” Mr. Trump.

The accounts again raised the question of why Mr. Bolton and other senior officials have refused to respond to congressional subpoenas seeking their testimony. “I believe that those who have information that the Congress deems relevant have a legal and moral obligation to provide it,” Ms. Hill said. What’s Mr. Bolton’s answer to that?

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