Microsoft raised alarms that Russian state hackers may be targeting the campaign of U.S. Democratic presidential candidate Joe Biden.
According to three sources, Microsoft alerted SKDKnickerbocker, a Washington-based campaign advisory and strategy firm working for the Biden campaign, that hackers with suspected Russian government ties tried to infiltrate its network.
But the hackers failed, a source said, because the firm was well-defended.
The Kremlin called the hacking report "nonsense" and has long denied other reports of interference in U.S. elections.
One source said it wasn't clear whether Biden's campaign was the target, or whether the Russians wanted information about other SKDK clients.
Representatives of Microsoft and SKDKnickerbocker declined to comment to Reuters for this story. A Biden campaign spokesperson did not respond.
American spy services believe Russia was behind the 2016 cyberattacks on Democratic Party servers as part of an effort to undermine presidential candidate Hillary Clinton and help Republican Donald Trump.
And U.S. intelligence agencies have raised the alarm about Russian and other foreign government attempts to interfere in November's vote.
But a stunning allegation this week from a former U.S. official accused the Trump administration of suppressing that threat.
A former top aide at the Department of Homeland Security filed a whistleblower complaint alleging he was pressured by political appointees at DHS - including acting secretary Chad Wolf - and national security adviser Robert O'Brien to stop providing assessments of Russian election interference.
The whistleblower says he was asked to hold one such notification because it could "make the president look bad."
Representatives of the White House and DHS flatly denied the allegations.