WASHINGTON — Nancy Pelosi, the House minority leader, sent a letter on Tuesday to Speaker Paul D. Ryan urging him to bar Republicans from using hacked documents in the campaign, arguing that “defending our democracy is more important than any advantage or disadvantage in this election.” Ms. Pelosi, calling the hack into Democratic servers, apparently by Russians, “an unprecedented assault on the sanctity of our democratic process,” said Mr. Ryan should not condone either party using materials originating from the cybercrime. “Democrats and Republicans must present a united front in the face of Russia’s attempts to tamper with the will...