Under pressure from the government and right-wing thugs, civic activists are banding together to protect their vital work.

Fears that April 1 would mark the beginning of new efforts to malign and discredit anticorruption activists in Ukraine have turned out to be well founded. That date was the deadline—under a new law supported by the government—for anyone in civil society working to counter corruption to declare their income and assets publicly.

The first shot in the smear campaign was fired against Iryna Fedoriv, editor of the anticorruption website Chesno (Honesty). In an apparent case of paid news, an online outlet that is widely considered to be under the influence of the Radical Party published a story about Fedoriv’s property, implying that she had acquired it through corrupt means because the price she paid was too low. Fortunately, her critics missed their mark: Fedoriv bought the property in 2006, when the Ukrainian currency was far more valuable. This slipup with the facts was not out of character for the Radical Party, a populist faction that is prone to publicity stunts.

Nevertheless, high-level politicians did not give up their efforts to thwart anticorruption organizations and activists in Ukraine, peddling misleading claims aimed at casting a shadow over their work. For example, President Petro Poroshenko’s representative in the parliament, Iryna Lutsenko, has reiterated her conviction that forcing anticorruption activists to declare their assets was the right step, accusing them of unprincipled self-promotion.

Such attempts to discredit civic activists are part of a larger…