* Talks stall on scope of crop subsidy reform and cuts
* No fallback plan - milk prices could double in January
* USDA's Vilsack urges nonstop work on five-year bill
* Senators say put our bill on any must-pass package
WASHINGTON, Dec 13 (Reuters) - On the same day a bipartisan bloc of senators called for speedy passage of the long-overdue U.S. farm bill, House and Senate negotiators blamed each other for a stalemate on how to cut crop subsidy spending by one-third.
Agriculture Secretary Tom Vilsack, meanwhile, warned that there was no alternative to enacting a new five-year law, such as passing a temporary extension.
Milk prices in U.S. grocery stores could double in January without a new bill, since without reauthorization U.S. farm policy would revert to the provisions of the last "permanent" farm bill, the Agriculture Act of 1949.
Negotiators are deadlocked on the size of potential cuts in food stamps for the poor, the largest U.S. anti-hunger program and the bulk of USDA's spending.
All the same, an agreement on crop subsidies is the linchpin to passing the farm bill.
"We're very close. There are only a few differences but they are significant," Chairwoman Debbie Stabenow of the Senate Agriculture Committee said, adding that the leaders of the House Agriculture Committee must improve their "unacceptable" offer.
In reply, the House negotiators said they were waiting for a "balanced offer" from the Senate.
The House and Senate farm bills would cut crop subsidy spending by one-third, or roughly $20 billion, over 10 years. But they disagree sharply over how far to go in reforming the network of farm supports that date from the Great Depression.
SKIP THE HASSLE, USE OUR BILL, SAY SENATORS
A group of 33 senators signed a letter to Democratic Majority Leader Harry Reid and Republican Leader Mitch McConnell saying passage of a new farm bill as soon as possible is vital and the solution is to enact the Senate-passed bill.
"Accordingly, we urge you to considering folding the Senate's strong bipartisan bill in any end-of-year package," the letter said.
Agricultural leaders in Congress say proposed farm bill cuts, ranging from $23 billion to $35 billion, could be part of a deficit-reduction package to avert the "fiscal cliff" of automatic tax increases and spending cuts next month.
"They should be working 24/7 until they get a resolution," said Vilsack to reporters. "And then hope there is a (legislative) vehicle it can be attached to. That train could leave the station any day, at any moment."
Aides to Stabenow said there was no work on their side to prepare a stopgap bill to keep farm programs running until a five-year bill is passed. Vilsack also said an extension of the now-expired 2008 farm law was unlikely.
A POLICY DISPUTE AND A REGIONAL FIGHT
In part, the Senate-House deadlock is a dispute over how to revamp U.S. farm supports for an era of volatile market prices and tight supplies. It has elements of a regional dispute between the corn- and soybean-growing Midwest and the South, home to cotton, rice and peanuts.
Southerners say the Senate bill favors the Midwest with its plan to compensate growers when crop revenue is below average, and that insurance-like programs are ill-suited to their crops. Midwestern groups say the House bill allows unduly high supports for Southern crops and would put them at a disadvantage.
Stabenow said the Senate offered 75 percent of the spending on wheat, peanuts and rice that was demanded by the House. "We have done everything we can," she said after meeting committee members privately to discuss the negotiations.
In a joint statement, House Agriculture Chairman Frank Lucas and Minnesota Rep. Collin Peterson, the Democratic leader on the panel, said "this is not a rice, peanut, and wheat issue. Rather, it's about making sure policy is defensible to taxpayers and works for all commodities in all regions of the country."
Also on Thursday, Mississippi Sen. Thad Cochran indicated he may try to replace Kansas Sen. Pat Roberts as the Republican leader on the Agriculture Committee in 2013. Roberts was a strong backer of the Senate-passed farm bill.