O'Rourke spoke Tuesday for 50 minutes to a crowd of hundreds in a Penn State auditorium, promising a grassroots campaign and taking questions, including one accusing him of spouting platitudes, instead of actual policy positions.
He insisted he's being specific about where he stands, such as advocating for universal pre-kindergarten. Later, he waded through hundreds more fans outside who sought autographs, selfies or handshakes.
Many who came to see him say they are drawn by his youth and his rising star reputation.
O'Rourke made the stop in Pennsylvania on his way from Ohio to the early voting state of New Hampshire.