Sen. Claire McCaskill alleged on Thursday that Russian hackers have tried to gain access to her office’s computer network but have not succeeded.

The alleged attempt is one of the first public acknowledgments of a cyberattack tied to the midterm elections. McCaskill, a Missouri Democrat, is seeking a third term representing a state that voted for Republican President Donald Trump two years ago, and she is among the most at-risk Senate Democrats in the upcoming midterms.

“While this attack was not successful, it is outrageous that they think they can get away with this. I will not be intimidated,” McCaskill said in a statement from her office calling Russian President Vladimir Putin “a thug and a bully.”

The Russian embassy in Washington did not immediately respond to a request for comment outside usual business hours.

The campaigns of three political candidates in this year’s U.S. midterm elections have been targeted in phishing attacks, Microsoft said this month. The company declined to identify the candidates or say who it believed was behind the attacks.

The attempt against McCaskill’s office was a variant of the password-stealing technique used by Russian hackers against John Podesta, the chairman of Hillary Clinton’s 2016 Democratic presidential campaign, the Daily Beast reported earlier on Thursday.

Special counsel Robert Mueller charged 12 Russian intelligence officers this month in connection with a bitcoin-funded hacking of Democratic organizations and of the Clinton campaign.