Sharing a debate stage for the first time, Rick Perry, right, and Mitt Romney sparred in a series of testy exchanges over jobs, Social Security and the proper tone of a candidate who presumes to lead the country. The session at the Ronald Reagan Presidential Library in Simi Valley was the first to include the Texas governor, who supplanted Romney as the presumptive GOP front-runner, and was the focal point throughout most of the evening -- though not always happily. At one point, the oft-targeted Perry compared himself to a piñata.