Aetna and Humana are scrambling to convince antitrust regulators in the U.S. Department of Justice that their proposed $34 billion merger won't hurt consumers by reducing competition in the health insurance market. 

Until recently, officials at both companies expressed confidence that the deal would be approved, but the prospect of rejection appears to have increased dramatically in the past month, as consumer groups, political leaders and state and federal regulators have publicly and privately voiced concerns about the effects of such an enormous acquisition. 

Perhaps the most significant display of opposition to the deal came from California Insurance Commissioner Dave Jones, who urged the feds last month to block the merger, citing the reduced competition it would create for millions of consumers in the nation's largest state.