Senator Claire McCaskill (Democrat-Missouri), who is up for reelection this year, late on July 26 confirmed a report on the Daily Beast website saying Russia's GRU intelligence agency tried to break into McCaskill's computers in August.
The Daily Beast report came after a Microsoft executive said last week that the company had helped stop e-mail phishing attacks on three unidentified U.S. congressional candidates.
In a statement, McCaskill said she wants to hold the hackers and Russian President Vladimir Putin accountable.
"While this attack was not successful, it is outrageous that they think they can get away with this," she said. "I will not be intimidated. I've said it before and I will say it again, Putin is a thug and a bully."
Her office would not give any details about the attempted attack or say how they learned about it.
McCaskill's state of Missouri overwhelmingly voted for President Donald Trump in 2016 -- which makes her one of the most vulnerable senators up for reelection this year.
According to The Daily Beast, the e-mail phishing scam that targeted her office was similar to a successful Russian hacking of Democrat Hillary Clinton's presidential campaign chairman, John Podesta, in 2016.
Senator Mark Warner (Democrat-Virginia), the top Democrat on the Senate Intelligence Committee, said the news confirms what he and others have warned for some time.
"The Russians saw 2016 as a success and they'll be back in 2018 unless we do far more to protect ourselves than we're currently doing," he said.
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