By David Morgan and Gram Slattery WASHINGTON (Reuters) – If U.S. Senate candidate Evan McMullin succeeds in unseating Republican Senator Mike Lee of Utah, he could occupy a uniquely powerful role as an independent ready to work with either party in the narrowly divided chamber. While maverick Democratic Senators Joe Manchin and Kyrsten Sinema have used their positions in the 50-50 chamber to block parts of President Joe Biden’s agenda, an independent McMullin could wield even more influence if Republicans pick up one — and only one — other seat in the Nov. 8 midterm election. That would crea…