One of the nation’s top rating agencies has downgraded its outlook for Maine bonds because of the state’s continuing budget problems, especially Medicaid spending, and its lack of reserves.

Moody’s Investors Service on Friday lowered its outlook from stable to negative for the state’s $498 million in general-obligation bonds.

The company maintained its rating of Aa2, third-highest, on the debt and assigned the same to $55.8 million of general-obligation bonds the state plans to issue, Moody’s said Friday in a statement. Maine plans to issue the debt on May 31 for capital projects, Moody’s said.

Moody’s notes in the statement that Maine’s employment growth was for all intents and purposes flat in 2011, compared to a national growth rate of 1.1 percent. The agency also pointed to problems with Maine’s demographics, as well as potential issues with how cuts to state spending may affect specific parts of the economy.

“A weak demographic profile, including slow population growth, an aging work force, and out-migration of younger residents will challenge the state over the medium term,” Moody’s wrote. “While the health care jobs have been an economic driver over the course of the recent recession, the state’s efforts to reduce spending on social services, especially Medicaid, may reduce future growth prospects for that sector.”

The rating agency said the negative outlook reflects Maine’s “recurring challenges” on the spending side of its budget, “primarily in the Department of Health and Human Services which includes Medicaid.” The agency also saw problems with minimal budget stabilization fund balances — commonly known as a rainy day fund — and a “weak general fund liquidity position reflecting the lack of reserves.”

Charles Colgan, an economist with the University of Southern Maine’s Muskie School of Public Service, said this move by Moody’s is really a signal more than anything else.

“Practically, I don’t think it means very much — it’s really a signal to bond investors that one day in the future the rating might be downgraded — but it doesn’t say anything about whether or when that may happen,” said Colgan.

Colgan noted that Moody’s is concerned about the ongoing DHHS issues, demographics and other challenges that are not new.

“What they’re basically saying is if you want to invest in the state of Maine’s bonds, be aware these sorts of things are going on when you price them,” said Colgan.

So will this mean Maine pays more to borrow money through bonding? Colgan said the state isn’t doing much bonding now, though the Legislature has just approved a $96 million package. If the bond proposals pass the governor’s desk and are later passed by voters in November, the earliest they’ll go on sale is early winter, Colgan said.

“Between now and then, a lot can happen,” he said.

Colgan, who served 12 years in the Maine state planning office in the past, including a stint as Maine State Economist, said in years past, when these sorts of warnings were sounded, state officials would make trips down to New York to talk to the bond raters and make sure they understood the Maine economy.

“This was a long time ago — in those days, Moody’s and Standard & Poor’s tended to look at the Maine economy as nothing more than lumber, fish and potatoes. We had to go down and do a fair amount of educating about the realities of the Maine economy,” said Colgan. “It may be that state officials want to tell a different story to the raters before they do their next review.”

In a release Friday, State Treasurer Bruce Poliquin said his office over the last several months has lead a team of state government officials in discussions with the national rating agencies to update Maine’s credit rating.

“I’m pleased that Moody’s Investors Service has affirmed Maine’s solid Aa2 credit rating. This rating will continue to give investors confidence in the quality and security of our general obligation bonds. Our office anticipates strong demand at the May 31 bond sale,” Poliquin said in the release. “I appreciate the helpful guidance from Moody’s as Maine continuously strives to improve its credit rating. I note that Moody’s recognized the positive financial impact of state government eliminating $1.7 billion of our unfunded public pension liability last year. This year, the rating agency acknowledges the long-term financial health of our ongoing initiative to right-size our Medicaid program.”

In the Moody’s statement, the rating agency noted that Maine’s revenue performance is “tracking slightly over budget” while the state’s unemployment rate of 7.2 percent is below the nation’s 8.1 percent.

The rating reflects the state’s “manageable debt levels; improving revenue performance; the resolution of recent budget shortfalls with largely recurring actions; and pension reforms that have improved the state’s funded ratios and lowered the annual required contribution,” Moody’s said. “Debt ratios are below the 50-state medians and debt is scheduled for rapid retirement within 10 years.”

BDN writer Matt Wickenheiser contributed to this report.