Russian intelligence agents targeted Sen. Claire McCaskill Claire Conner McCaskillMomentum growing among Republicans for Supreme Court vote before Election Day Democratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally MORE (D-Mo.) with an attempted breach ahead of the 2018 midterm elections, The Daily Beast first reported Thursday.

The news site reported that McCaskill, considered among the most vulnerable senators up for reelection, was one of three congressional candidates a Microsoft executive said last week had been targeted by hackers.

“Russia continues to engage in cyber warfare against our democracy,” McCaskill said in a statement. “I will continue to speak out and press to hold them accountable. While this attack was not successful, it is outrageous that they think they can get away with this. I will not be intimidated. I’ve said it before and I will say it again, Putin is a thug and a bully.”

The report comes shortly after special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE indicted 12 Russian intelligence officers in the hacking of the Democratic National Committee in 2016, the latest charges filed in his ongoing probe of Moscow's election interference.

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McCaskill, whose home state of Missouri went to President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE by double-digits, would be the first confirmed target of attempted Russian hacking in the 2018 midterms.

The Daily Beast reported that the hackers targeted Senate staffers through phishing emails, falsely telling them that their Microsoft Exchange passwords had expired and sending them to a mock-up of the Senate’s Active Directory Federation Services login page.

Microsoft seized one of the fake sites in October and redirected traffic from the domain to its own server. This allowed the company to see who was sent to the mock site.

“Earlier this year, we did discover that a fake Microsoft domain had been established as the landing page for phishing attacks,” Tom Burt, Microsoft’s vice president for security and trust, said at the Aspen Security Forum last week.

Burt said that Microsoft had blocked the attempted breaches, and the Daily Beast reported that the attempts against McCaskill's team were ultimately unsuccessful.

Tokyo-based cybersecurity group Trend Micro told The Hill earlier this year that it had identified websites designed to look like the U.S. Senate’s internal email system and that they were part of an email-harvesting operation.

The sites were reportedly created by the Russian hacking group known as “Fancy Bear,” which is linked to the 2016 DNC hack. The Daily Beast reported that the techniques used by the Fancy Bear hackers were similar to those used on McCaskill’s staff.

The Daily Beast noted that the attempted breach against McCaskill took place in August 2017, around the time Trump took a trip to Missouri, where he urged attendees at a rally to vote McCaskill out of office.

“We must lower our taxes and your senator, Claire McCaskill, she must do this for you, and if she doesn’t, you have to vote her out of office,” Trump said at the time.

Last week, amid fierce criticism of his friendly press conference with Russian President Vladimir Putin, Trump tweeted that Russia might interfere in the upcoming midterm elections to help Democratic candidates.

I’m very concerned that Russia will be fighting very hard to have an impact on the upcoming Election. Based on the fact that no President has been tougher on Russia than me, they will be pushing very hard for the Democrats. They definitely don’t want Trump! — Donald J. Trump (@realDonaldTrump) July 24, 2018

Speaking next to Trump in Finland, Putin said that he had hoped the president would beat Democratic nominee Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE in 2016.

"Yes, I did, because he was the one who wanted to normalize relations with Russia,” Putin said when asked if he wanted Trump to win.

— Updated at 10:17 p.m.