Putin Not a Friend, Poisoned Russian Activist's Wife Tells Trump Vladimir Kara-Murza is fighting for his life in a Moscow hospital tonight.

 -- The wife of a poisoned Russian opposition activist says President Donald Trump must not consider Vladimir Putin a friend.

In an exclusive interview with ABC News after visiting her comatose husband in a Moscow hospital, Evgenia Kara-Murza said, “[Trump] must know that such people as Vladimir Putin are not friends. And they cannot be dealt with on friendly terms.”

Her husband, 35-year-old Vladimir Kara-Murza, an outspoken critic of Putin, remains in critical condition, fighting for his life, she said.

His doctors say he was poisoned by an unknown substance, she added. The activist was hospitalized on Thursday shortly after feeling ill; within hours, most of his major organs were failing. It is the second time in two years that he was poisoned; the first time, in 2015, left the father of three with nerve damage that left him walking with cane.

Evgenia Kara-Murza said that she does not know who is behind the poisoning but that she believes it can only be related to his work. A veteran opposition campaigner, he has appeared repeatedly before the U.S. Congress, pressing for it to impose sanctions on Russian officials involved in human rights abuse.

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On Sunday, Fox News Host Bill O’Reilly asked Trump if he respects his Russian counterpart, to which the U.S. president said he did but “that doesn’t mean I’m going to get along with him.”

When the host interjected that “Putin’s a killer,” Trump said, “Lot of killers. We’ve got a lot of killers. What, you think our country’s so innocent? You think our country’s so innocent?”

Wiping away tears during an interview from Moscow, Evgenia Kara-Murza told ABC News’ Brian Ross that neither her husband nor other Putin opponents will be stopped by the ugly tactics.

“People like my husband will never stop, no matter what. They will fight, with all of their might.”

Vladimir Kara-Murza is hardly the first Putin critic to have come to serious harm in mysterious circumstances.

“There is a pattern of Putin’s critics being poisoned or dying of mysterious, unexplained medical problems,” said former White House national security official Richard Clarke, now an ABC News consultant. “This is straight out of the old KBG playbook, and it’s death by poison.”

Two years ago this month, Boris Nemtsov, a prominent opposition leader in Russia and longtime critic of Putin, was shot to death on a Moscow bridge. Vladimir Kara-Murza worked closely with Nemtsov, who was the godfather of one of Kara-Murza’s daughters.

In 2006 a former Russian spy, Alexander Litvinenko, died after being poisoned by a rare radioactive material in London — a murder blamed on Putin by a British government-ordered inquiry.

Trump’s defense of Putin has drawn the ire not just of foreigners living in fear of the Russian president.

On Monday night, leaders from both U.S. political parties were at a loss to explain why the U.S. president continued to defend one of the United States’ starkest adversaries.

In an interview with ABC News from Capitol Hill today, Sen. John McCain, R-Ariz., was critical of the president’s comments, saying “The United States of America has made serious mistakes — we all know that — but not, nowhere near anything like the intentional murders committed by Vladimir Putin.”

Meanwhile, Rep. Nancy Pelosi, D-Calif., said that she wanted “to know what the Russians have on Donald Trump.”

“I think we have to have an investigation by the FBI into his financial, personal and political connections to Russia,” she said.

ABC News’ Cho Park, Alex Hosenball and Paul Blake contributed to this report.