MOSCOW — The Ukrainian police on Friday questioned Ukraine’s former president, Petro O. Poroshenko, after threatening to arrest him if he failed to appear, in what a growing chorus of critics including the European Union say is a politically motivated vendetta.

They say the failure of the United States to discourage the new Ukrainian government’s legal assault on Mr. Poroshenko illustrates how the once-dominant American role and moral authority in Ukraine’s internal politics have been undermined by President Trump’s efforts to persuade the country’s new leaders to investigate Mr. Trump’s political opponents.

Political prosecutions have haunted Ukraine since independence. A former president, Viktor F. Yanukovych, in 2011 imprisoned the opponent he narrowly beat in a presidential race, Yulia V. Tymoshenko, and other opponents.

Mr. Poroshenko is not just a former president but also a prominent political opponent of President Volodymyr Zelensky, who won election last spring on an anti-corruption platform that included a pledge to end politically motivated investigations.