AURORA — Beto O’Rourke repeated his recent call to round up high-powered assault-style weapons and pushed for stronger gun-control measures Thursday during an outdoor town hall in this Denver suburb, but rarely has his attention to the issue resonated so deeply in a crowd. O’Rourke’s visit to Aurora, his first to Colorado since he joined the small army of Democrats angling to challenge President Donald Trump in the 2020 election, took place near the site of the 2012 movie theater mass shooting.