Two Republican senators unveiled a Medicare overhaul Thursday that features an accelerated transition to private health insurance for many seniors, a gradual increase in the eligibility age, and higher premiums for middle-class and upper-income retirees. Sens. Tom Coburn of Oklahoma and Richard Burr of North Carolina say they're not out to win a political popularity contest. Instead, they want to engage fellow policymakers and the public in a "grown-up" conversation about the scope of changes needed to preserve Medicare in some form for future generations.