NEW YORK — In one of his first acts as President Obama's new health-care adviser, Chris Jennings traveled to Harlem last month to pay a visit to his old boss, Bill Clinton. Armed with a PowerPoint presentation detailing how the new health-care law will go into effect, Jennings made his pitch: Obama needs your help, both to persuade millions of uninsured Americans to sign up for coverage and to combat Republican attempts to undermine the law. Clinton tried and failed to enact an expansion of health-care coverage as president. Now Obama, who signed the Affordable Care Act into law, is relying on Clinton to help make it a legacy.