A senior aide to President Barack Obama suggested Friday that Sen. Rand Paul would be the greatest threat to Democrats’ hopes to retain the White House in 2016. Speaking to reporters, counselor to the president Dan Pfeiffer said the Kentucky Republican is “one of the most intriguing candidates” in the field because of his appeal to younger voters of both parties. “He’s the only Republican I think who has articulated a message that is potentially appealing to younger Americans,” Pfeiffer said at a breakfast organized by the Christian Science Monitor.