WASHINGTON (MCT) — While Mitt Romney and Paul D. Ryan are campaigning on a promise to “preserve and protect” Medicare, their proposal to revamp the popular government health insurance program would be the plan’s biggest gamble since it was created nearly a half-century ago.

The members of the Republican presidential ticket argue that giving seniors vouchers to shop for a private insurance plan would spark competition among health insurers, holding down costs and ensuring the long-term viability of Medicare.