MOSCOW — From a certain perspective, certainly the Kremlin’s, Vladimir Kara-Murza’s behavior in Washington could be seen as treasonous, a brazen betrayal of his homeland.

In a series of public meetings on Capitol Hill, Mr. Kara-Murza, a leader in the Russian opposition, urged American lawmakers to expand economic sanctions against the Russian government under a law known as the Magnitsky Act. That would hasten political change in Russia, he argued.

Back in Moscow a month later, in May 2015, the changes Mr. Kara-Murza detected were going on in his own body. Midway through a meeting with fellow dissidents, beads of sweat inexplicably dotted his forehead. His stomach churned.

“It all went so fast,” he recalled. “In the space of about 20 minutes, I went from feeling completely normal to having a rapid heart rate, really high blood pressure, to sweating and vomiting all over the place, and then I lost consciousness.” He had ingested a poison, doctors told him after he emerged from a weeklong coma, though they could find no identifiable trace of it.