The Democratic National Committee (DNC) has sued US president Donald Trump's campaign, his son, his son-in-law, the Russian Federation and WikiLeaks, claiming they conspired to help Mr Trump win the 2016 presidential election.
The DNC filed the lawsuit today in federal court in Manhattan.
It seeks unspecified damages and an order to prevent further interference with DNC computer systems.
The lawsuit claimed Mr Trump and his associates had relationships with Russia that enabled the creation of a Trump-Russia conspiracy.
The DNC said Russia "mounted a brazen attack on American democracy", beginning with a cyber attack on its computers.
It said the hack penetrated DNC computers and phone systems and extracted tens of thousands of documents and emails.
Mr Trump has said repeatedly there was no collusion between his campaign and Russia.
DNC chairman Tom Perez said in a statement: "During the 2016 presidential campaign, Russia launched an all-out assault on our democracy, and it found a willing and active partner in Donald Trump's campaign."
He called it an "act of unprecedented treachery".
The lawsuit also accuses Donald Trump Jr of secretly communicating with WikiLeaks, saying the president's son was offered a password to an anti-Trump lawsuit in one exchange.
The lawsuit blames the president too, saying he praised the illegal dissemination of DNC documents throughout the autumn of 2016, making it a central theme of his speeches and rallies.
The lawsuit said Mr Trump's son-in-law, Jared Kushner, was part of the conspiracy as a senior adviser and key decision-maker in the campaign. It said he began in June 2016 to control all campaign data-driven efforts, beginning with the establishment of a 100-person "data hub" in San Antonio, Texas, and by hiring Cambridge Analytica, the social media and analytics firm.
The lawsuit said the conspiracy "inflicted profound damage upon the DNC", costing it donations and preventing it from communicating the party's values and vision to voters, as well as sowing discord inside the Democratic party itself.
"The conspiracy constituted an act of previously unimaginable treachery: the campaign of the presidential nominee of a major party in league with a hostile foreign power to bolster its own chance to win the presidency," the lawsuit said.
"And, in carrying out this effort, defendants' conspiracy to disseminate documents stolen from the DNC in violation of the laws of the United States as well as the laws of the state of Virginia and the District of Columbia. Under the laws of this nation, Russia and its co-conspirators must answer for these actions."