(CNN) A federal judge presiding over a historic civil lawsuit in Ohio against drug distributors, pharmacies and retailers for their roles in the opioid crisis on Thursday denied a motion to step down on grounds he has shown bias.

US District Court Judge Dan Polster, who is hearing the case in Cleveland, questioned why lawyers for seven of 22 defendants waited more than 18 months to question his impartiality based on comments he made at hearings, media interviews and public forums.

The landmark trial is set to begin next month. It combines nearly 2,000 cases involving cities, counties, communities and tribal lands, accusing opioid makers of causing the epidemic.

"The undersigned is confident that the imminent trial of the first bellwether case, along with the parties' ongoing settlement negotiations -- addressing not only the bellwether trial, but also global resolution -- will continue to bear fruit," Polster wrote in his decision.

"And the undersigned is confident that no reasonable person can legitimately question my impartiality."

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