Obama could hardly be surprised that he's faced pressure from all sides as he tries to pass comprehensive health care reform. But in his push to expand Medicaid, he's run into resistance from a more unexpected quarter: governors.

The health care bills in both the House and Senate would expand Medicaid by upping the eligible poverty rate and adding new categories of eligibility. Some estimates have as many as 20 million new beneficiaries nationally, adding $500 billion in federal costs to the program.

States have shared the cost of Medicaid since the program’s inception in 1965, and have control of their own programs and eligibility rules. For states already reeling from the recession, the burden of tens of thousands more Medicaid recipients could break the bank. Under Medicaid's current plan, the federal government covers 57 percent of costs, leaving the other 43 percent up to the states.

Complaints about expansions to the program were front and center at last month's National Governors Association meeting. They're also responsible for turning a number of state executives firmly against the prospect of expanding Medicaid without more federal assistance, creating yet another roadblock for Obama's plans.

"Adding new eligibility categories certainly gives us pause," said Joy Johnson Wilson, the health policy director at the National Conference for State Legislatures. "We don't know how to finance that."

Having states shoulder a portion of the cost of Medicaid has become increasingly difficult; the program already represents a huge share of state budgets and has been growing by 11 percent annually. The economic stimulus package in February included $87 billion in Medicaid assistance, but states still find themselves short.

"The combination of the economic downturn and the rise in need for public coverage leaves states in a real fiscal bind," said Diane Rowland, the executive vice president of the Kaiser Family Foundation. "Independent of reforms, states have a legitimate concern about their ability to sustain programs without federal assistance."

To try to start the discussion, a group of governors including Christine Gregoire of Washington and Haley Barbour of Mississippi went to D.C. in June. The July meeting of the National Governors Association also included private meetings with Health and Human Services Secretary Kathleen Sebelius. The concerns have begun reaching lawmakers as well. On CNN's "State of the Union" in June, Democratic Sen. Dianne Feinstein said she would have trouble supporting a bill that would expand Medicaid costs in California, where budget problems have been well-documented.

"If you change the Medicaid rate, for example, it has an impact on California between $1 billion and $5 billion a year," Feinstein said. "Now how could I support that? It would take down the state."

Under most plans, the federal government would cover the cost of new eligible patients, but the question is for how long. The Senate HELP committee plan would roll the costs back to the states over five years, which leaves some states dreading the cutoff.

"Nobody knows what the fiscal situation is going to be in five years, and states have a lot more limitations on ways they can raise funds," said Rowland.

Because states have different eligibility requirements and methods of delivery, there are also concerns that federal assistance won't help all states equally. The federal assistance would only apply to new eligibles, so a state that already covers, say, individuals up to 133 percent of the poverty line would get less help than those only covering up to 100 percent. Those new requirements would also remove some of the independence and control states have enjoyed over their Medicaid delivery systems.

These concerns could derail the promise, warned Wilson.

"We want to make it happen, but we also don't want to promise something we can't deliver," she said. "They have to help us deliver it, because we as states can't do it now."