WASHINGTON — Armed men in masks and tactical gear have shown up at secure ballot drop boxes. Candidates of both parties have been physically attacked, election workers intimidated. And threats against members of Congress are up tenfold. For many voters, a vicious spiral of violence and fear is creating angst, paranoia and an overwhelming sense of dread that the nation is on the eve of destruction, according to a growing body of public opinion research. Democrats worry that the GOP is bent on seizing power regardless of the outcome of elections — a concern rooted in former President Donald Trump’s...