Raley's supermarket and union officials are back at the bargaining table, this time with help from a federal mediator, to try to stave off the local grocery chain's first strike.

After more than a year of failed negotiations, the two sides have until Saturday midnight to work out their differences, or as many as 7,000 employees could walk out, cutting the company's labor force in half.

In a demonstration of solidarity, a couple hundred union members and their supporters rallied in front of the Federal Building in Sacramento Thursday. Union leaders then turned their attention to San Francisco and the last-ditch negotiations that will play out there over the next two days.

The long-fought labor dispute was kicked into high gear this week when Raley's tried to impose its final contract offer on employees beginning Thursday, a move that was met with a firestorm of union protest. A federal mediator intervened, and Raley's agreed to delay through Saturday. If there's no deal, and the old contract isn't extended, the United Food Commercial Workers could walk out Sunday morning.

"We'll either be at the finish line or on the picket line," said Mike Henneberry, communications director for UFCW Local 5, headquartered in San Jose. Local 5 and Local 8 in Roseville are leading union talks.

The Sacramento-based company employs 13,000 people and operates about 130 stores in Northern California and Nevada, including Raley's Superstores, Bel Air Markets and Nob Hill Foods. The strike would be the first in the grocery chain's 77-year history

"This is the furthest it's ever gotten," said Raley's spokesman John Segale.

Raley's wants to freeze wages and eliminate premium pay for Sunday and holidays, a proposition union leaders say is a deal-breaker because employees will see big cuts in their take-home pay. Segale said the premiums are an outdated practice from the 1950s and '60s, when employers had to offer incentives to get workers to come in on a Sunday. In this economy, he said, it doesn't make sense. Employees will still earn double-time pay on holidays.

"Honestly, we're not that far apart, and it's certainly not worth going out on strike," he said.

Segale said stores would remain open during a strike and the company's 6,000 nonunion employees would fill in the gaps. He expects some employees will resign from the union and continue to work.

Stocking produce at the Nob Hill Foods supermarket in Walnut Creek on Thursday, Alfredo Minor said he's anxious about losing pay. But he's one of the employees who will swallow their losses and picket.

"We're hoping that it's not going to happen," he said. "It's not good for anybody. But if we have to, we have to. We can't let them take everything away from us."

Contact Heather Somerville at 925-977-8418. Follow her at Twitter.com/heathersomervil.