President Obama urged a group of young activists Wednesday not to give up on promoting his signature health-care law, as enrollment picked up on HealthCare.gov. Roughly 27,000 Americans signed up for insurance on the federal exchange on Tuesday, according to internal figures, bringing the site's three-day enrollment total to 56,000. That figure is more than double the number who enrolled online in the entire month of October, which was almost 27,000. Buoyed by the Web site's improved performance, Obama and his aides have been touting the importance of signing up for plans before Dec. 23, the deadline for obtaining insurance scheduled to start Jan. 1.