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The country's mayors are urging congressional leaders and the administration to avoid far-reaching federal spending cuts set to take effect in January, warning that America's cities will suffer if Washington fails to act.

The U.S. Conference of Mayors released a letter Thursday signed by more than 130 mayors calling sequestration -- the $1.2 trillion in federal spending cuts scheduled to begin taking effect this January -- "perhaps the biggest threat to our metro economies."

The cuts are the result of last year's failed negotiations over a plan to reduce the federal deficit. Congress approved a plan last year that gave it a powerful incentive to reduce the deficit by $1.2 trillion. If they failed to do so on their own, then by law, automatic spending cuts of $1.2 trillion split evenly between defense and non-defense spending would take effect from 2013 to 2021.

The thinking was that sequestration was so unpalatable that it would force both political parties to return to the negotiating table and hammer out an agreement. But so far, no agreement to avoid sequestration has been reached, and the cuts are scheduled to take effect in just a few months.

Mayors say Congress is avoiding its responsibility and that cuts could devastate cities by taking away important grants they use to pay for transportation projects and first responders, among other things.

"We all know the lame duck session is upon us, but we can't afford to wait and sit around," Sacramento Mayor Kevin Johnson said. "We're asking Congress to act now."

According to the mayors, a third of the cuts to non-defense discretionary spending will affect state and local programs, including those dealing with education, housing, health, public safety and the environment. That, in turn, would force "inevitable" cuts to local services by states and localities.

All total, for programs affected by sequestration, the cuts scheduled to take effect in January would equal around 7.8 percent in FY 2013, according to Federal Funds Information for States, which tracks federal spending levels on grants that affect state and local governments.

Mayors highlighted programs that could face challenges as a result of the cuts. Johnson decried potential cuts to education programs. Philadelphia Mayor Michael Nutter warned that his city would have to scale back on a program that cleans dangerous lead from homes. Atlanta Mayor Kasim Reed said the cuts could jeopardize his city's streetcar project. Salt Lake City Mayor Ralph Becker said the cuts would affect Community Development Block Grants, used to aid low- and moderate-income families. And Minneapolis Mayor R.T. Rybak said he worries about the impact the cuts could have on firefighter staffing levels.

Rybak said his city is using grants that provided funding for Minneapolis to bring a new class of recruits through its fire academy. But, as it stands, he doesn't know if those grants will continue in the future. "What is a mayor supposed to do? Do you hire that recruit class?" Rybak said.

The city has moved forward with hiring, "but we have to be looking over our shoulder," he said.

Mayors say that the scheduled cuts are also inserting uncertainty into their budgeting process.

Nutter, president of the U.S. Conference of Mayors, urged Congress to take a balanced approach and develop a plan that includes new revenues in addition to defense and non-defense cuts as a way to achieve that goal. As it stands, sequestration relies exclusively on spending cuts. "We will oppose any balancing proposal that does not include ... new sources of revenue," Nutter said.

Several mayors characterized Congress's unwillingness thus far to raise revenue as a cost-shift from the federal government onto cities. "We're going to make it known that cuts, without revenue, isn't a cut really at the federal level," Reed said. "It will mean that those burdens and those responsibilities will go to cities that are already constrained,"

Meanwhile, although Washington hasn't shown much progress in addressing those mayors' concerns, there's little disagreement that sequestration could have a serious impact.

The non-partisan Congressional Budget Office recently warned that sequestration, along with the scheduled expiration of tax cuts, "will lead to economic conditions in 2013 that will probably be considered a recession."

Last week, the Office of Management and Budget released a report outlining the impact of the scheduled sequestration cuts. It called sequestration a "blunt and indiscriminate instrument" that should be avoided.