WASHINGTON—In the wake of a series of lapses in presidential security, Secret Service director Julia Pierson appeared before the House Oversight and Government Reform Committee Tuesday, seemingly unaware that President Obama himself was, at that moment, being chased by an assailant in the background. “We have made mistakes, and we want to assure the American people that we are committed to fixing them,” Pierson said, as a frantic Obama ran back and forth through the congressional hearing room, occasionally clambering over chairs to escape what observers described as a highly agitated man with a knife. “Protecting the president is our most important duty, and you have my promise that from this moment forward, we will do it as we should.” At press time, Obama was seen blocking knife blows with a legal pad as Pierson declared her intention to conduct a thorough internal review.