There is no evidence that our diversity is our strength yet every lefty spouts this mantra ad nauseam.
The Left's dream of power is that their ideological juju will be so persuasive, so overweeningly overweening, that people(s) with widely divergent interests, backgrounds, and understandings of life will rush together to support the "overarching" Leftist objective of more, and still more, and still more again, secular power -- hell, let's say it, it's ideologically-based Empire, with the advantage of numbers, i.e. of supporting population base, over the old European empires supported by at best one nationality each.
The Russian Bolsheviks ate and slept that propaganda for generations -- and yet, when Gorbachev left office, the whole enterprise, the Russian and Soviet Empire, split apart into previously-subordinate ethnicities, some of them Russian (Ukraine, Byelorussia aka Belarus). It doesn't work, and yet libs and Progs continue to push it here in the States, with better historical support inasmuch as the U.S. is a compacted, not a national or ethnic, society.