They met behind closed doors over the past four months in the Capitol Hill office of California's senior senator, carving out details of what could be a landmark change for agriculture-based immigration.

Now, however, California growers and farmworkers have hit an impasse, and they are running out of time to agree on a program that could affect hundreds of thousands of farmworkers nationwide.

"Our window is closing quickly," said Tom Nassif, president of the Western Growers Association, which temporarily pulled out of talks with the United Farm Workers after both sides refused to budge on minimum wages and other regulations for a new temporary visa program.

The stalled talks threaten to leave agriculture out of the comprehensive immigration bill expected next week from a bipartisan Senate group. But U.S. Sen. Dianne Feinstein, D-San Francisco, said in an emailed statement Friday "we are close to a resolution, and it is my intention to wrap this up next week."

"We got to a point where we didn't see much give on their side," Nassif said of farm labor groups. "They didn't see much give on our side. So we took a little break."

The months-long talks had reached broad consensus on reforms that would legalize hundreds of thousands of agricultural workers living here illegally.

A new "blue card" would give them a chance to become legal residents and eventually citizens. It would offer a faster process than the 13-year one proposed for other illegal immigrants.

But field workers' newfound mobility and freedom to work wherever they want would likely draw some to the cities for better jobs, compelling agricultural companies to look elsewhere for workers.

"That's fine, but we need to have a good guest-worker program when that happens," Nassif said.

The talks fell apart because "grower associations are insisting to Congress that farm workers are currently paid too much," said United Farm Workers spokeswoman Maria Machuca.

Federal surveys reveal average wages -- $9.17 for vegetable-sorters, $10.62 for ranch hands, $19.90 for inspectors -- but growers say the surveys need to be redone before being used to set minimum wages.

"What we're suggesting is we have a good statistical study that tells us what the wage should be," Nassif said.

The United States has a 10-month agricultural work visa, the H-2A, but California growers say it is too costly and slow for an unpredictable industry. California has the nation's biggest farm workforce but ranks 13th in H-2A workers. Most of the 55,000 guest-workers recruited in 2011 went to Southern states.

Farm business leaders want a smoother, nimbler program and more visas. Unions want to keep or strengthen the existing program's workplace and wage protections, and argue that growers are trying to pay less than what farmworkers make now.

Pete Aiello is too busy gearing up for strawberry season to keep up with politics, so the co-owner of Gilroy-based Uesugi Farms said he was surprised to hear of the stalled negotiations.

Aiello has never used guest-workers, though he would consider an "easier-to-manage and less costly" program. He also said he is not too worried about legalized farmworkers moving off the farms.

"If we want these folks to stay here, to come here to work, we have to be prepared to compete."