A bipartisan group of U.S. lawmakers marked the two-year anniversary of Kremlin critic Vladimir Kara-Murza’s imprisonment by calling for his immediate release. 

Kara-Murza, who lives in solitary confinement in a Siberian maximum-security prison, was sentenced to 25 years last April for treason and other related charges as Russian authorities continue their crackdown on domestic dissent.

The Moscow City Court claimed Kara-Murza was guilty of “high treason“ for “disseminating knowingly false information about the Russian Armed Forces” when he delivered a speech to the Arizona House of Representatives in 2022 that criticized Russia’s invasion of Ukraine.

His sentence is the longest term handed down to a political prisoner in the post-Soviet era.

Sens. Ben Cardin, D-Md., and Roger Wicker, R-Miss., co-led a group of 80 bipartisan lawmakers urging the Biden administration to declare the Russian dissident as “unlawfully and wrongfully detained.”

Fox News Digital obtained a letter sent to Secretary of State Antony Blinken from Senate Foreign Relations Committee Chair Cardin and other lawmakers demanding Kara-Murza’s release and the aforementioned designation.

“There is little time left to end the ongoing and unjust detention of U.S. Legal Permanent Resident and Russian dissident Vladimir Kara-Murza,” the letter read in part. “Mr. Kara-Murza’s family has grave concerns that he may not survive much longer. His situation is even more perilous following the killing of Alexei Navalny. Mr. Kara-Murza is the most prominent imprisoned democracy activist still alive in Russia.”

The State Department referred Fox News Digital to spokesperson Matthew Miller’s remarks on Kara-Murza’s two-year imprisonment anniversary but did not provide specifics when asked about efforts to give the Russian opposition leader the designation sought by U.S. lawmakers.

“The Department of State continuously reviews the circumstances surrounding the detentions of U.S. nationals overseas, including those in Russia, for indicators that they are wrongful. When making assessments, the Department conducts a legal, fact-based review that looks into the totality of the circumstances for each case individually,” a spokesperson said.

Russian human rights lawyer and the Center for European Policy Analysis’ Democracy Fellow Grigory Vaypan told Fox News Digital that Kara-Murza is now Russia’s “prisoner number one.” 

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“He’s definitely political prisoner number one on Putin’s list, and his life is certainly in danger now that we see with the murder of Navalny that Putin’s regime demonstrates to the world that it’s willing to kill political prisoners in Russia,” Vaypan said. 

He added that Kara-Murza, who was reportedly poisoned twice in 2015 and 2017 by agents of the Russian state, is essentially on “Putin’s death row.” 

“His health is deteriorating. He has never fully recovered from the effects of those two poisonings. Now, he is not only in prison, he’s on solitary confinement, which is basically indefinite. He can be in his tiny prison cell for many months, and with the effects of those two poisonings, his health is getting worse,” Vaypan explained. “This is why it would be fair to say that he’s essentially on Putin’s death row now.”

Memorial, Russia’s oldest human rights group, counts roughly 700 political prisoners in Russia today. 

Political prisoners are further isolated and punished in an effort to prevent them from continuing to speak out against the Russian authorities. They can be put into solitary confinement, deprived of food, mail, phone calls with relatives or family visits. 

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“There’s a wide array of those measures that the Russian prison authorities can resort to. And we’re increasingly seeing that, especially after the beginning of Russia’s full-scale invasion [of] Ukraine, we’ve seen more people jailed for exercising their right to free speech,” Vaypan told Fox News Digital. “And we’ve seen an increasing number of people being further harassed and pressured even while in prison.”

Kara-Murza’s wife, Evgenia, reflected on the deaths of other Russian opposition figures like Alexei Navalny and Boris Nemtsov at the hands of the Putin regime.

“[They] target the most courageous, the most principled, those Russians who risk not only their freedom but very often their lives to show you that Russia can be different,” she said at an event on Capitol Hill.

“As my husband put it, and I quote, ‘It is my hope that when people in the free world today think and speak about Russia, they will remember not only the war criminals who are sitting in the Kremlin but also those who are standing up to them because we are Russians too.’”