Aetna, one of the nation's biggest health insurers, staked a $5.7 billion claim in the burgeoning market for government-funded coverage Monday when it announced plans to buy fellow insurer Coventry Health Care.

The Hartford, Conn., company's proposed cash-and-stock acquisition of Coventry will bolster its Medicaid enrollment months before millions of uninsured Americans are expected to become eligible for coverage through that state-federal program as part of President Barack Obama's massive health care overhaul.

The deal also ramps up Aetna Inc.'s Medicare Advantage and Medicare prescription drug businesses, as interest in these plans grows while the baby boomer generation ages and becomes eligible. Medicare Advantage plans are privately run versions of the government's Medicare coverage for the elderly and disabled people.