ONE DAY after President Trump and Vladi­mir Putin held their first phone call, Russian-backed forces mounted their largest offensive in months in eastern Ukraine. Now, days later, one of Russia’s most prominent opposition activists is in a coma in a Moscow hospital, where he was rushed after suddenly taking ill on Thursday morning. Vladi­mir Kara-Murza, a writer and civil- society activist with many supporters in Washington, is believed by his family to be the victim of a poisoning attack — the second they believe he has suffered since 2015.

His agony most likely holds a message from Mr. Putin to the new Trump administration. Since 2014, the Kremlin has endured sanctions from the United States and the European Union for its aggression in Ukraine and for human rights violations, such as the killings of lawyer Sergei Magnitsky and opposition leader Boris Nemtsov. With the new assault on Ukraine and the felling of Mr. Kara-Murza, the Kremlin hopes to establish that such crimes will be tolerated by the new U.S. president as part of a refounded relationship with Moscow.

So far, Mr. Putin’s gambit is succeeding: Mr. Trump, while sparring with close U.S. ally Australia, has had nothing to say about the events in Ukraine and Moscow.

Mr. Kara-Murza offered an ideal target for Mr. Putin’s challenge. The 35-year-old former journalist, a fluent English speaker, was a close associate of Mr. Nemtsov. His family lives outside Washington, and his support in Congress can be intuited from the statements that poured out Thursday from, among others, Rep. Steny H. Hoyer (D-Md.), Sen. Benjamin L. Cardin (D-Md.) and Sen. John McCain (R-Ariz.).

The Kremlin will claim it has nothing to do with Mr. Kara-Murza’s sudden illness, just as it denies responsibility for the rockets raining down on Ukrainian army positions. But poisoning has become a well-established menace to Kremlin opponents in the Putin era. One case where near-certainty has been established is that of former KGB agent and Putin critic Alexander Litvinenko, who according to an exhaustive official British investigation was poisoned by Kremlin agents in London in 2006. Half a dozen other poisoning cases are suspected, including of a former president of Ukraine.

Mr. Kara-Murza first suffered from an apparent poisoning attack in May 2015, shortly after he testified in Congress in favor of expanding human rights sanctions on Russia. Suddenly taken ill during a meeting, within hours he was in a Moscow hospital’s intensive care unit, his organs failing. Doctors there saved his life but were unable to explain what had afflicted him; tests in France later detected an unusual level of metals in vital organs.

With stunning courage, Mr. Kara-Murza returned to Moscow and resumed his political activities after regaining just enough strength to walk with a cane. When asked if he were not an obvious target for a Kremlin hit, the dissident invariably replied that he believed he had no choice but to go on working for democracy and human rights in his country.

Mr. Kara-Murza is the sort of freedom fighter that the United States has always defended. He walks in the footsteps of Andrei Sakharov and Natan Sharansky, the Soviet-era dissidents whom President Ronald Reagan fought to save. If Mr. Trump and Secretary of State Rex Tillerson now do nothing on his behalf, they will show that their administration is ready to appease Mr. Putin at the price of American values.