Washington ? President Barack Obama urged reluctant lawmakers Saturday to quickly approve nearly $50 billion in emergency aid to state and local governments, saying the money is needed to avoid “massive layoffs of teachers, police and firefighters” and to support the still-fragile economic recovery.

In a letter to congressional leaders, Obama defended last year’s huge economic stimulus package, saying it helped break the economy’s freefall, but argued that more spending is urgent and unavoidable. “We must take these emergency measures,” he wrote in an appeal aimed primarily at members of his own party.

The letter comes as rising concern about the national debt is undermining congressional support for additional spending to bolster the economy. Many economists say more spending could help bring down persistently high unemployment, but with Republicans making an issue of the record deficits run up during the recession, many Democratic lawmakers are eager to turn off the stimulus tap.

“I think there is spending fatigue,” House Majority Leader Steny Hoyer, D-Md., said recently. “It’s tough in both houses to get votes.”

Democrats, particularly in the House, have voted for politically costly initiatives at Obama’s insistence, most notably health care and climate change legislation. But faced with an electorate widely viewed as angry and hostile to incumbents, many are increasingly reluctant to take politically unpopular positions.

Last month, the House stripped Obama’s request for $24 billion in state aid from a bill that also would extend emergency benefits for jobless workers. Senate Majority Leader Harry Reid, D-Nev., has said he hopes to restore that funding, but with debate in that chamber set to resume this week, Democrats say they have yet to assemble the votes for final passage. And neither chamber has mustered support for another Obama priority: $23 billion to avert the layoffs of as many as 300,000 public school teachers.

Senior Democratic aides said the White House is partially at fault because it has not made additional spending on the economy a clear priority.

In recent weeks, the White House has appeared more intent on cutting spending — calling for the sale of vacant buildings, threatening to veto a defense bill over a jet engine project the Pentagon views as unnecessary and urging every agency to come up with a list of low-priority programs for elimination. Obama has also proposed a three-year freeze in discretionary spending unrelated to national security, an idea endorsed by leaders of both parties at a meeting at the White House last week, according to White House chief of staff Rahm Emanuel.

With the letter, however, Obama makes a direct and unequivocal case for additional “targeted investments,” including state aid and several less-expensive initiatives aimed at assisting small businesses, and he asks lawmakers to be patient on the deficit.

“It is essential that we continue to explore additional measures to spur job creation and build momentum toward recovery, even as we establish a path to long-term fiscal discipline,” Obama wrote. “At this critical moment, we cannot afford to slide backwards just as our recovery is taking hold.”