US officials have formally accused Russia of cyber attacks against political organisations in order "to interfere with the US election".
Recent hacked emails are "consistent with the methods and motivation of Russia-directed efforts", the Department of Homeland Security said.
Data revealing discussions within the Democratic Party was hacked earlier this year.
Some states reported "probing" attempts made on "election-related" systems.
However, officials said those attempts could not be directly linked to the Russian government.