Lawyers for former President Donald J. Trump on Thursday joined an effort to disqualify Fani T. Willis from leading the election interference case against Mr. Trump in Georgia, on the grounds that she created a conflict of interest by hiring her romantic partner to help prosecute the case.

Mr. Trump’s lawyers also raised a new argument for ousting Ms. Willis, the Fulton County district attorney: that she violated state bar rules when she claimed in a recent speech that racism was behind the effort to remove her.

The relationship claim surfaced on Jan. 8 in a filing from Michael Roman, one of Mr. Trump’s co-defendants in the Georgia case. Six days later, Ms. Willis, who is Black, gave a speech at a church in Atlanta in which she suggested that her critics were “playing the race card” by criticizing her hiring of the special prosecutor, Nathan J. Wade, who is also Black.

Ms. Willis has neither confirmed nor denied a relationship with Mr. Wade, though she has been ordered to provide a written response to Mr. Roman’s motion by next week.

The filing on Thursday from Mr. Trump’s lawyers called Ms. Willis’s comments “a glaring, flagrant and calculated effort to foment racial bias into this case by publicly denouncing the defendants for somehow daring to question her decision to hire a Black man.”

Ms. Willis’s statement, the lawyers wrote, violated a section of the state bar’s rules of conduct for Georgia prosecutors that instructs them to “refrain from making extrajudicial comments that have a substantial likelihood of heightening public condemnation of the accused.”