He left Russia last week to go through rehabilitation treatment abroad after what his lawyer described as a second acute poisoning.
Vladimir Kara-Murza, a Russian human-rights champion, also urged US President Donald Trump's administration to keep pressure on Moscow, warning that ignoring evidence that Russia interfered in last year's US election would only hand the Kremlin an invitation to ramp up its domestic oppression and its aggression abroad.
"For too many years, for too long, leaders of Western democracies have been just ignoring and moving on from what Mr Putin has been doing," Kara-Murza told a Senate appropriations subcommittee.
Continuing to do so would signal "weakness, lack of any kind of will, I would think, and invitation (for Russia) to carry on" with its aggressive activities, he added.
The 35-year-old Kara-Murza was an ally of the late opposition politician Boris Nemtsov, who was shot dead close to the Kremlin in 2015.
He works as federal coordinator for the Open Russia foundation of Mikhail Khodorkovsky, a former oil tycoon who served a decade in jail after openly opposing Putin.
Kara-Murza was hospitalized in 2015 and diagnosed with acute kidney failure in connection with poisoning. Tests found high levels of heavy metals in his blood.
During his Senate appearance he showed no outward signs of his poisoning. At one point Kara-Murza rose to kiss his wife, who he said helped cared for him after he fell into a coma earlier this month.
"Sometimes there are near misses" in Russian political murders, "and one happens to be sitting before you," Kara-Murza told the panel.
He stressed it was "vital" for the United States to stay engaged with Russia's civil society, including by maintaining public diplomacy programs and supporting the work of human rights groups.
And he urged Washington to continue to apply the Magnitsky Act, passed by Congress in 2012, which to date has sanctioned some 40 Russian nationals due to serious human rights violations.
"This is not only about money. Much more importantly it's about the message the US sends to Russia's civil society," he added. "Do you choose to engage or to turn away?"
Kara-Murza left Russia last week to go through rehabilitation treatment abroad after what his lawyer described as a second acute poisoning that nearly took his life.
Senator Lindsey Graham, who chairs the subcommittee, blasted Putin's administration as thugs who know bounds. "They will kill, they will steal, they will do whatever's necessary to stay in power," he said.
Graham said he wanted Congress to consider funding and establishing a "counter-Russia account to help frontline states and organizations who are fighting back against Putin's regime."