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From their right-wing challengers to climate concerns, Presidents Biden and Lula will have a lot of familiar ground to cover when they meet at the White House on Friday. But perhaps the most important issue facing the leaders of the world's two biggest presidential democracies will be convincing their own nations, and the world, that democracy is alive and well. The BBC's Nomia Iqbal and Mariana Sanches explain how the leaders and their nations compare at this critical moment.