Hackers stole $US2.3 million ($A3.3 million) from the Wisconsin Republican Party's account that was being used to help re-elect US President Donald Trump in the key battleground state, the party's chairman says.
The party noticed the suspicious activity on October 22 and contacted the FBI on Friday, Republican Party Chairman Andrew Hitt said.
Hitt said the FBI is investigating.
FBI spokesman Leonard Peace did not immediately return a message seeking comment.
"There's no doubt RPW is now at a disadvantage with that money being gone," Hitt said.
The party and campaign needs money late in the race to make quick decisions, he said.
Hitt said the hackers were able to manipulate invoices from four vendors who were being paid to send out direct mail for Trump's re-election efforts and to provide pro-Trump material such as hats that could be handed out to supporters.
Invoices were altered so when the party paid them, the money went to the hackers instead of the vendors, Hitt said.
Hitt said it appears the attack began as a phishing attempt.
Trump won Wisconsin by fewer than 23,000 votes in 2016 and it remains a key swing state this year.
Trump planned his third visit to the state in a week on Friday.
Democrat Joe Biden also planned to campaign in Wisconsin on Friday.
Australian Associated Press
