It is the second time in two years that Vladimir Kara-Murza, 35, has been hospitalized in grave condition, and his wife, Evgenia, claims the Kremlin is to blame.
"The Russian government and President Putin are responsible for what happened to my husband two years ago, and now, one way or another, I'm not saying that they're the ones who did that, but they've created such a climate in our country, that actually encourages this kind of behavior," Evgenia said.
CNN cannot independently confirm her claims, and Russia has denied any connection to Kara-Murza's illness.
"It's pure nonsense to make any connection of this unfortunate case with President Putin," Kremlin spokesman Dmitry Peskov told CNN.
Doctors in the hospital where Kara-Murza is being treated say they still "have no real idea" what caused his illness, but agree that it was triggered by a "toxic substance," his lawyer Vadim Prokhorov told CNN. He said he has no direct proof his client was poisoned.
Doctors have diagnosed Kara-Murza with "acute intoxication by an unidentified substance," Evgenia said, adding that the risks are now more serious due to his previous illness in 2015.
Kara-Murza is still too weak and ill to speak, but has been communicating by blinking his eyes, Evgenia said. As of Thursday, he began breathing on his own.
Evgenia 'terrified' every time husband returns to Russia
Kara-Murza, who lives with his wife and three children in Virginia part-time, was in Russia to promote a documentary on the assassination of his late friend, the former leading opposition figure Boris Nemtsov.
Nemtsov was shot to death on a bridge near the Kremlin
as he walked home from a restaurant in February 2015. Kara-Murza became ill four months later.
Evgenia told CNN that she is "absolutely terrified" every time her husband goes back to Russia to do opposition work, but that he is a "Russian patriot" committed to his cause.
Deputy leader of the Freedom Party, Kara-Murza is also part of the Open Russia foundation, an organization of anti-Putin activists who call for open elections, a free press and civil rights reforms.
'My major organs began to fail'
CNN spoke to Kara-Murza in 2015, months after his illness nearly killed him.
"I fell into a coma, all of my major organs began failing, one after another. It was the kidneys first, then the lungs, the heart, the liver," he said at the time.
He said he believed his 2015 suspected poisoning was a politically motivated attack.
"Frankly there is no other possible reason," he said. "I don't have any money dealings. I don't have any personal enemies. I didn't steal anybody's wife."
US lawmakers urge investigation
Kara-Murza's hospitalization has triggered an outcry from US lawmakers.
US Sen. Marco Rubio urged
the White House and the State Department to question Russian authorities about the case.
"Vladimir Putin does not deserve any benefit of the doubt here, given how commonplace political assassinations and poisonings have become under his regime," said Rubio, a frequent Putin critic. "I am praying that Kara-Murza's condition improves."
Speaking about Kara-Murza on Tuesday, US Sen. John McCain delivered a thinly-veiled rebuttal
to recent comments by US President Donald Trump that appeared to draw a moral equivalence between the US and Russia.
"Vladimir (Kara-Murza) knew that Putin is a killer. And he's a killer. And he might very well be the next target. Vladimir knew there was no moral equivalence between the United States and Putin's Russia," said McCain, who has been a fierce critic of Trump's rhetoric on Russia.
McCain's remarks were made after Trump appeared to defend Putin
in an interview with Fox News host Bill O'Reilly, who described Putin as a "killer."
"There are a lot of killers," Trump said. "You think our country's so innocent?"
Putin's spokesman called on Fox News to apologize.
O'Reilly responded by telling the Kremlin to "check in with me around 2023."
The incident comes amid chatter that a potential meeting between Trump and Putin may be on the cards. "There's nothing certain," Peskov, the Kremlin spokesman, told journalists on a conference call Thursday.
"There is a common understanding that the priority can be and should be the fight against terrorism and the Islamic State. As for other issues, we will work on them after we understand the format of the meeting," Peskov said.