The 18- to 34-year-olds who helped elect Barack Obama could consign his signature domestic policy achievement to failure.

That’s because not enough millennials have signed up for Obamacare to make it work well. Despite repeated outreach — including entreaties from all manner of celebrities, including NBA stars and Obama himself — young people make up less than 30 percent of Obamacare customers. he White House had set a goal of 40 percent in that age bracket to sustain a healthy marketplace because millennials tend to be healthier and, therefore, balance the costs of sicker, older customers.

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