Bernie Sanders is fighting to oust two Hillary Clinton supporters heading key Democratic National Convention committees, despite the party’s refusal to remove them, in the latest sign of disunity amid a bitter fight for the presidential nomination.

An attorney for Mr. Sanders’s campaign sent a letter to Democratic National Committee officials on Friday asking them to disqualify former congressman Barney Frank and Connecticut Gov. Dannel Malloy, who are co-chairmen of the convention’s rules and platform committees, respectively.

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