ATLANTA—Sidney Powell, a former legal adviser to Donald Trump, pleaded guilty to misdemeanor charges on Thursday in exchange for a sentence that means no jail time, agreeing to testify for the prosecution in the sweeping Georgia racketeering case against the former president and more than a dozen other defendants. 

Fulton County District Attorney Fani Willis’s August indictment alleged that Trump and 18 other co-defendants participated in a “criminal enterprise” to subvert the 2020 presidential election.