The judge in the Paul Manafort trial reminded Special Counsel Robert Mueller's team on Friday of the high bar for conviction -- that they must prove the former Trump campaign chairman knowingly violated tax and bank laws related to his political work overseas.

On the trial's fourth day, U.S. District Court Judge T.S. Ellis III indicated he believes the prosecution has demonstrated Manafort had control of foreign bank accounts despite checking a box on tax returns saying he didn’t have them.

But Ellis reminded prosecutors, “The government has to prove that [Manafort] knew what the requirement was and that he deliberately violated it.”

Ellis also reminded prosecutors they have to prove Manafort's “willfulness” in failing to file a Report of Foreign Bank and Financial Accounts.

Manafort, facing charges of bank and tax fraud related to his political work in Ukraine, has pleaded not guilty to the charges.

MANAFORT STRUGGLED TO PAY BILLS IN 2016, BOOKKEEPER TESTIFIES

A source close to Manafort’s team told Fox News the defense has not yet decided whether to have Manafort testify during the trial. This person said they will make the decision after the prosecution rests its case, which could be next week.

Friday’s testimony has focused on the tax fraud charges. The years involved, 2010 to 2014, were before Manafort worked for President Trump's campaign. Though the case stems from Mueller's probe of election meddling by Russia and possible collusion by the Trump campaign, there has been no mention of election interference.

Manafort served as chairman of Trump’s presidential campaign during the summer of 2016, and was forced out in August of that year amid news stories about his ties to then-Ukrainian President Viktor Yanukovych.

On Friday, Philip Ayliff, a CPA who prepared Manafort’s taxes, testified Manafort signed tax returns that showed “none” checked for any foreign bank accounts between the years 2010 and 2014.

The government pointed out that Manafort signed his tax returns “under penalty of perjury."

Prosecutor Uza Asonye walked Ayliff through each year and asked “what did Mr. Manafort say for this year for foreign bank accounts?”

Ayliff responded “none” for each one.

The prosecution also showed documents where the accountant emailed Manafort asking if he had foreign accounts, and Manafort responded that he didn't.

But Mueller's team says Manafort repeatedly used accounts in Cyprus to pay for an array of luxury items and services.

Prosecutors have introduced a host of exhibits and are in the process of calling several witnesses as part of their effort to paint Manafort as a tax scofflaw who failed to report money spent on luxury items -- then lied to get bank loans when his foreign consulting work dried up.

The prosecution has released exhibit photos of Manafort's expensive clothes, including a $15,000 ostrich coat and other pricey suits.

On Thursday, Manafort’s bookkeeper, Heather Washkuhn of the firm NKFSB, testified that Manafort racked up large bills but struggled to pay them in 2016.

“Did there come a time in 2016 when Mr. Manafort had trouble paying his bills?” prosecutor Greg Andres asked the accountant in federal court in Alexandria, Va.

“Yes,” Washkuhn replied.

The Manafort defense hinges on convincing jurors that Manafort was the boss who didn’t handle bills or accounts, but left that to his deputy, Rick Gates, who is cooperating with prosecutors and may testify as its star witness.

“Rick Gates had his hand in the cookie jar and couldn’t let his boss find out,” Manafort defense attorney Thomas Zehnle said during opening arguments.

Ellis, a 78-year-old Reagan-appointed judge in the case known for colorful comments, said this week the prosecution cannot prove a key part of their case unless prosecutors call Gates to the stand.

Fox News’ Anne Ball, Peter Doocy and The Associated Press contributed to this report.