GOP senators question exchanges' solvency

"User fees will be determined based on the budgetary requirements of operating and maintaining the exchange," the lawmakers wrote to Treasury Secretary Jack Lew and Health Secretary Kathleen Sebelius.

"The federal debt is nearly $17 trillion and the cost of the [exchange tax credits] has increased over 107 percent … To better understand whether the administration is implementing the law with federal spending in mind, please provide a detailed response to [our] questions."

The Affordable Care Act requires state insurance marketplaces, scheduled to launch on Oct. 1, to be financially self-sustaining starting in 2015.

The administration says the exchanges will open for enrollment on time. The related user fees are designed to accompany an expected flood of new policyholders for insurance companies as the individual mandate takes effect.

The letter came from Senate Finance Ranking Member Orrin Hatch (Utah), Richard Burr (N.C.), Pat Roberts (Kan.), Johnny Isakson (Ga.), Chuck Grassley (Iowa) and Mike Enzi (Wyo.).