Republican-led efforts to re-examine last fall's vote are spreading as experts and election officials warn that the proliferation amounts to a grave threat to U.S. democracy. At the center of the push is Arizona, where the private company hired by the Republican-controlled state Senate continues its review of more than 2 million Maricopa County ballots, despite prior audits finding no evidence of fraud. With former President Donald Trump and others on the right following that count closely — despite it having no legal ability to overturn the result — GOP officials and voters have pushed for similar probes in at...