Key Democrats introduced legislation Wednesday that would allow people 50 and older to buy into Medicare, saying it’s an incremental step to universal coverage that’s stage-ready and easier to pass than more ambitious liberal plans. They also said it’s sorely needed, since Americans aged 50 to 64 tend to develop medical issues and get “knocked around” by private insurers. Some constituents are even putting off an early retirement because they must cling to their job-based insurance, according to Sen. Debbie Stabenow, Michigan Democrat, who filed the bill with Sen. Tammy Baldwin of Wisconsin. “So many people say they’re holding their...