The federal judge in the trial of ex-Trump campaign chairman Paul Manafort told Special Counsel Robert Mueller's team on Thursday they cannot prove a key part of their case unless prosecutors call Manafort’s former business partner to the stand.

U.S. District Court Judge T.S. Ellis III issued the warning after prosecutors suggested Wednesday that Rick Gates – who is cooperating with prosecutors and had been considered a potential star witness – might not be called to the stand after all.

Referencing Gates, Ellis told prosecutors in court they “can’t prove conspiracy without him.”

“Not necessarily,” Mueller prosecutor Greg Andres responded, before saying they still have “every intention” to call Gates to the stand.

Ellis, a 78-year-old Reagan-appointed judge known for colorful comments, also quipped that his “wife wasn’t fooled” by the prosecution’s comments on Wednesday leaving open the option of not calling Gates — suggesting he did not believe the prosecution was being serious about the possibility.

MUELLER PROSECUTORS REBUKED REPEATEDLY BY TRIAL JUDGE WITH A HISTORY OF COLORFUL RULINGS

During opening arguments this week, the defense team made it clear they intend to blame Gates, who handled some day-to-day business operations for Manafort, for many of the alleged reporting deficiencies Manafort is charged with.

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

Before the jury entered the courtroom on Thursday, there was renewed discussion about whether pictures of Manafort’s clothing and other expensive purchases could be presented during the trial. Ellis repeated that he will not allow the prosecution “to gild the lily.”

“It could become relevant if there was a dispute of what the money was spent for,” Ellis said, suggesting there is no dispute at this time.

Manafort’s bookkeeper, Heather Washkuhn, also testified Thursday that she had no knowledge that Manafort had any foreign holdings.

When asked by prosecutor Greg Andres if that would be important for her to know, Washkuhn replied, “yes.”

Washkuhn also said her firm charged Manafort about $100,000 a year and that she went to Gates when she couldn’t reach Manafort.

Prosecutors have introduced a bevy 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.

But the Mueller team was rebuffed by Ellis on Wednesday when it first tried to introduce photos of Manafort’s closets, filled with suits and other high-end articles of clothing. Ellis noted that those photos would eventually become fodder for the media, and called them "unnecessary" for jurors to see.

“Enough is enough. We don’t convict people because they have a lot of money and throw it around,” he said.

In between periodic disputes with the judge, a new batch of witnesses was being brought to the stand on Thursday.

Michael Regolizio from New Leaf Landscaping business testified that his business was paid more than $400,000 between 2010 and 2014 for landscaping work at Manafort’s home in Bridgehampton. Regolizio said Manafort paid by check or international wire transfer from overseas.

The trial had been expected to last three weeks. But Ellis surprised the courtroom late Wednesday when he said, "I’m hoping to finish this case much sooner than anyone predicted.”

Fox News’ Peter Doocy, Jake Gibson and Gregg Re and The Associated Press contributed to this report.