State party chairman Andrew Hitt said that the suspicious activity was noticed on 22 October and that the FBI is now investigating.
Wisconsin is a key swing state in the 2020 election, having been won by Mr Trump in 2016 by just 23,000 votes.
“There’s no doubt RPW is now at a disadvantage with that money being gone,” Mr Hitt said, explaining that the party and campaign needs money late in the race to make quick decisions.
The Associated Press reports that Hitt said the hackers manipulated invoices from four vendors who were being paid to send out direct mail for the Trump campaign.
They were also providing pro-Trump material such as hats to hand out to supporters.
Hackers altered the invoices to direct payments to themselves and not the vendors. Mr Hitt said that it appears the attack began as a phishing scam.
Both the president and Democratic candidate Joe Biden will campaign in the state on Friday. It will be Mr Trump’s third visit to the state this week.
The RealClear Politics polling average for Wisconsin has Biden ahead by 6.4 points.
An ABC News/ Washington Post poll released on Wednesday has him leading by 17 points.
With reporting from The Associated Press