Microsoft Corp (MSFT.O) recently alerted one of Democratic presidential candidate Joe Biden’s main election campaign advisory firms that it had been targeted by suspected Russian state-backed hackers, according to three people briefed on the matter.

The hacking attempts targeted staff at Washington-based SKDKnickerbocker, a campaign strategy and communications firm working with Biden and other prominent Democrats, over the past two months, the sources said.

A person familiar with SKDK’s response to the attempts said the hackers failed to gain access to the firm’s networks. “They are well-defended, so there has been no breach,” the person said.