Rep. Matt Gaetz
Rep. Matt Gaetz (R-FL) (Photo: Gage Skidmore)​

Diabolical, but brilliant.

That's how MSNBC legal analyst Danny Cevallos is describing federal prosecutors' decision to target Matt Gaetz's ex-girlfriend as they investigate whether the Florida GOP congressman sex-trafficked a minor.

Earlier this week, NBC News reported that Gaetz's ex-girlfriend testified before a grand jury as part of a possible immunity deal in the case, calling it "a major development that suggests the Department of Justice may be moving closer to indicting him."

On Friday, Cevallos wrote that while we don't know details about the testimony of Gaetz's ex-girlfriend, "we do know ... that if the target of a federal investigation is going to be romantically involved, prosecutors would prefer it be with a girlfriend rather than a wife."

"And they’d always prefer to deal with an ex-girlfriend than a current girlfriend," Cevallos wrote.

He explained that federal courts recognize two important marital privileges.

"Either spouse may assert the confidential marital communications privilege to prevent the other one from testifying about private discussions they have during the marriage," Cevallos wrote. "Additionally, the spousal testimonial privilege permits one spouse to refuse to testify against the other during the marriage."

But the privileges — which have their roots in "medieval" views that didn't recognize women as separate entities from their husbands — don't apply to girlfriends.

"It doesn’t matter if the girlfriend has lived with the boyfriend for years," Cevallos wrote. "It doesn’t matter if they have kids together. A girlfriend is not a wife — but she is a potential gold mine for prosecutors."

According to Cevallos, the only thing prosecutors like more than a cooperating girlfriend is a cooperating ex-girlfriend — but that preference is based on "human nature" and not "rules of evidence."

"An ex-girlfriend usually ex-likes the ex-boyfriend being investigated," he wrote. "Even better for the investigators, sometimes the ex-girlfriend hates the ex-boyfriend with a seething passion. It’s a lot easier for a witness to incriminate someone if they already hate their guts."