"Cybercriminals, using a sophisticated phishing attack, stole funds intended for the re-election of President Trump, altered invoices and committed wire fraud," Wisconsin Republican Party Chairman Andrew Hitt said in a statement.
The Republican National Committee reported the hack to the FBI and the state party said it's investigating. The FBI has not returned a request for comment. The story was first reported by the Associated Press.
The $2.3 million hit to Republican efforts comes just days before the election in a battleground state where Trump has consistently trailed Biden in the polls for months and where the state Republican party has trailed state Democrats in fundraising by over $35 million this cycle.
According to Hitt, hackers used a phishing scam to gain access to the state party system and then changed invoices and routing numbers so that money intended for vendors instead went to the hackers.
The state party discovered the stolen funds on Thursday, Oct. 22. The money that went to the hackers was supposed to go to vendors in charge of mailers and Trump campaign "swag," the party said.
Officials said the money stolen came only stolen from the Wisconsin GOP's federal account, which goes toward President Trump's reelection in the state.
"These criminals exhibited a level of familiarity with state party operations at the end of the campaign to commit this crime," Hitt said.
A separate account for statewide campaigns, which aids local, down-ballot races, was untouched, said Wisconsin GOP spokesperson Alec Zimmerman.
And no proprietary information or voter information was stolen, Zimmerman said.
Even before the $2.3 million loss, the Wisconsin GOP was tens of millions of dollars behind state Democrats, who have raised $59 million to the GOP's $23 million.
Hitt told the Milwaukee Journal Sentinel that it was "devastating" when they discovered the hack.
"Anytime you lose $2.3 million, I think that's probably an accurate way to describe it," Zimmerman said.
But the GOP maintained confidence in its efforts to reach voters, specifying that the products they'd intended to pay vendors for have already been distributed, though they haven't been paid for.
They'll now have to issue another $2.3 million to the vendors.
"While a large sum of money was stolen, our operation is running at full capacity with all the resources deployed to ensure President Donald J. Trump carries Wisconsin on November 3rd,” Hitt said.
The RNC, which said it will continue to aid the Wisconsin state party as the investigation continues, also projected confidence, despite the setback.
"The RNC never left Wisconsin after 2016, and we are confident that our ground game and the millions we are spending on TV and digital will deliver us another win there in 2020," RNC Communications Director Michael Ahrens said in a statement.
ABC News' Will Steakin contributed to this report.