Pressing for a final rush of health care enrollees, President Barack Obama said Tuesday that about 4 million people have signed up for health insurance through federal or state marketplaces set up under his health care law. But with a key deadline approaching fast, he urged some of his most steadfast backers to help sign up millions more by then. "We've only got a few weeks left. March 31st, that's the last call," Obama said, explaining that anyone not signed up by that date will have to wait until open enrollment begins anew in the fall. In the meantime, they risk being fined for not having coverage.