A federal judge in Chicago said Friday that indicted Cook County Commissioner William Beavers can argue at his tax evasion trial that he made an honest mistake when he failed to pay taxes on money from his campaign accounts — but only if he takes the stand himself.

The ruling from Judge James Zagel opens the door for defense lawyers to submit evidence that is crucial to their case.

Federal prosecutors asked Zagel to block that evidence during the trial, which starts Monday. They argue Beavers amended his tax returns and repaid some of the money he took only after he learned the feds were investigating him.

But Beavers’ lawyers argue those documents are key to understanding their client’s state of mind when he withdrew more than $226,000 from his three campaign accounts but didn’t report the income. 

And Zagel seemed to agree.

“We’re talking about things which can reasonably be thought to be a mistake,” Zagel said in a pretrial hearing. “It’s not like stabbing somebody in the chest with a hunting knife.”

But Zagel quickly added that any testimony about Beavers’ intent would have to come from the commissioner directly. Defense attorney Sheldon Sorosky wouldn’t say Friday whether he’d call Beavers to the stand.

Meanwhile, prosecutors revealed Friday that they plan to call workers from Horseshoe Casino in Hammond, Ind. as witnesses, to show Beavers lost a “lot of money” and gambled “very frequently” with money he allegedly took from his campaign accounts.

Assistant U.S. Attorney Matthew Getter alleged in court that during one trip to the casino, Beavers left twice to cash checks from his campaign accounts, then returned to to continue gambling. Getter said the prosecution plans to detail his gambling losses at trial to show jurors how Beavers was spending his money.

Zagel said prosecutors would have to be careful during the trial not to try to paint Beavers as a gambling “addict.”

Beavers, a longtime Chicago aldermen who was elected to the Cook County Board in 2006, was indicted in February for allegedly taking more than $226,000 in campaign money for personal use, neglecting to pay taxes on it, then filing fraudulent campaign finance reports to cover his tracks. He allegedly used nearly $69,000 of those funds to boost his aldermanic pension just before he left the City Council.

The indictment alleges Beavers also spent his $1,200 monthly commissioners’ stipend on personal expenses but didn’t report the money as income.

Jury selection is expected to begin Monday and opening arguments are slated for Tuesday.