Lawmakers are preparing for a second run at writing the new U.S. farm law that ended in a stalemate in 2012, and the biggest obstacle is not likely to be soil conservation or crop subsidies, but the billions spent mostly in cities and towns.

Analysts say food stamps for the poor, the biggest Agriculture Department program at an estimated $79 billion this year, is the make-or-break issue. Republicans are demanding far larger cuts than Democrats will entertain, and the debate is becoming increasingly partisan.

Enrollment in the program has doubled in a decade and costs have tripled. Critics say spending is out of control when only the neediest should get aid. Defenders say the weak economy is the culprit – enrollment is highest during economic turmoil – and that food stamps are targeted to avoid cuts in farm subsidies.

Food stamps “is the key to getting a final farm bill done. Not that there won’t be plenty of other fights,” said Pat Westhoff, director of the Food and Agricultural Policy Research Institute (FAPRI), a think tank at the University of Missouri.