Proud Boys leader Enrique Tarrio pleaded guilty to burning a Black Lives Matter banner in December.
His attorney filed a motion on Saturday to have the judge removed and to reduce Tarrio's sentence.
Tarrio was sentenced to five months in jail in August, and reported to jail on Monday night.
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The attorney for Proud Boys leader Enrique Tarrio filed a last-ditch motion on Saturday to reduce his client's five-month sentence for the burning of a Black Lives Matter banner belonging to a historically Black church in the nation's capital.
The incident, which Tarrio pleaded guilty to along with other charges, occurred in December before the January 6 insurrection at the Capitol.
Tarrio's attorney Lucas I. Danise said in the new filing that the sentencing judge in the case, Harold Cushenberry Jr., should be removed due to bias.
"My oldest daughter was actually baptized at that church, and I haven't really had any affiliation with the church," the judge said at Tarrio's plea hearing in July. "She's almost 40 now, and we attended with some regularity when she was young."
Cushenberry offered to recuse himself then, but Tarrio said, "that's not necessary."
At that July hearing, Tarrio, 37, pleaded guilty in D.C. Superior Court to one count of destruction of property and one count of attempted possession of a large-capacity ammunition feeding device.
The flag-burning took place during a December 12 rally supporting former President Donald Trump, for which Tarrio told Insider he was overseeing security.
Tarrio, who is Cuban American, has maintained that the Proud Boys - a recognized hate group linked to street fights and violence around the US - is not racist by nature, while recognizing that some members hold white-supremacist beliefs.
Tarrio's attorney Danise said in the Saturday filing that Tarrio did not know that the church owned the BLM flag. Danise said his client was denied due process as the government provided "ambiguous" evidence that showed Tarrio knew that the banner belonged to this church.
Danise also argues that his client's sentence is "unreasonably harsh and disproportionate when taking into account the goals of sentencing."