ONE OF the first foreign leaders to congratulate President-elect Joe Biden was Volodymyr Zelensky — and no wonder. President Trump unjustly persecuted the Ukrainian president, demanding that he launch a politicized investigation of Mr. Biden in 2019, while holding up U.S. military aid and a White House invitation. Mr. Biden, in contrast, has been one of Ukraine’s best American friends, visiting the country five times while vice president and strongly supporting its battles against Russian aggression and domestic corruption.

Mr. Zelensky now needs that support more than ever. A former comedian elected on an anti-corruption agenda, the 42-year-old president has recently seen that cause badly damaged by the Constitutional Court of Ukraine, which since August has issued several rulings hamstringing key reforms, including a registry of assets for government officials. Several of the judges were under investigation by the very anti-corruption agencies they neutered. According to Ukrainian experts, they also acted under the influence of Ukrainian oligarchs anxious to maintain the old networks of corruption.

Mr. Zelensky made some mistakes of his own, including pressuring the respected chief of Ukraine’s central bank to resign. Mr. Zelensky appeared to flounder in the search for responses to the court decisions. Then he contracted covid-19 and was hospitalized, though he has said his case is not serious. Meanwhile, the International Monetary Fund and the European Union are threatening to withhold badly needed aid because of the backsliding.

In short, the Ukrainian leader could use some help — and the incoming U.S. president is well-equipped to provide it. Mr. Biden pushed the previous Ukrainian government hard to crack down on corruption: His demand that a crooked prosecutor be fired was twisted into the false allegation that the then-vice president was trying to protect a company that hired his son as a board member. Mr. Biden also supported U.S. aid for Ukraine as it resisted Russia’s seizure of Crimea and invasion of two other eastern provinces.

Once inaugurated, the Biden administration could take several steps to strengthen Mr. Zelensky’s hand. One would be to prepare sanctions against the Constitutional Court judges who are complicit in tainted rulings; the Global Magnitsky Act provides for visa bans and asset freezes in such cases. A Biden Justice Department could also renew efforts to pursue criminal corruption cases against key Ukrainian oligarchs, including Dmytro Firtash and Ihor Kolomoisky, who have been instrumental in blocking reforms and in promoting Russian interests in Ukraine.

Most important will be for Mr. Biden to resuscitate the political alliance between Washington and Kyiv, which enjoys strong bipartisan support in Congress but was all but wrecked by Mr. Trump. Though he was induced to restore military aid to Ukraine, Mr. Trump continued to promote Russian disinformation that Ukraine rather than the regime of Vladimir Putin interfered in the 2016 U.S. election — and he never invited Mr. Zelensky to Washington. Correcting that vindictive omission should be one of Mr. Biden’s early foreign policy initiatives.

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