Russian government hackers breached Democratic National Committee computers to obtain opposition research on Republican Donald Trump, say officials.
The DNC's computer network was infiltrated in a "serious" incident, said chairwoman Debbie Wasserman Schultz.
A cyber security firm has been hired to help, she said, to "kick out the intruders and secure our network".
Russia has denied any involvement in the hacking.
"I completely rule out a possibility that the [Russian] government or the government bodies have been involved in this," Dmitry Peskov, a Krelim spokesman, told Reuters in Moscow.
The hack was first reported by the Washington Post.
DNC officials said the hack did not expose personal or financial information but email and chat traffic had been stolen.
The group says the hackers, two separated "sophisticated adversaries", have now been expelled from the system, and the breach was first discovered in April.
"Both adversaries engage in extensive political and economic espionage for the benefit of the government of the Russian Federation and are believed to be closely linked to the Russian government's powerful and highly capable intelligence services," cyber security firm CrowdStrike said.
Other organisations targeted by the Russian groups include presidential nominees Hillary Clinton and Donald Trump's networks and some Republicans political action committees, according to the Washington Post.
Russian president Vladimir Putin has spoken highly about Mr Trump, who has suggested the US pull out of Nato and described Mr Putin as a man he admires.
"The security of our system is critical to our operation and to the confidence of the campaigns and state parties we work with," Mrs Wasserman Schultz said.
"When we discovered the intrusion, we treated this like the serious incident it is and reached out to [cyber security group] CrowdStrike immediately."
Hackers from Russia were also accused of hacking into President Obama's unclassified emails in 2015.