The possibility of paying for groceries with your smart phone just moved one step closer to becoming a reality.

Eight months after competitors AT&T, Verizon and T-Mobile disclosed their unlikely partnership to create a “digital wallet” that allows users pay with the swipe of a smart phone, they announced their first major victory yesterday: All four major credit card companies have signed on.

Isis, the name of the mobile payment system, will now be supported by Visa, MasterCard and American Express — in addition to the project’s original partner, Discover. That means a customer can store virtually any credit card on an Isis-enabled phone and use it at checkout, creating one standard payment method. The system is scheduled to roll out in the first half of 2012 in two trial sites, Salt Lake City and Austin, Texas.

“Trying to get all the parties and their lawyers at the table to figure this out is a pretty Herculean task,” said Bob Egan, a mobility analyst and managing director for MGI Research in Boston. “From the payment perspective, this is the most comprehensive proposal out there.”

The decision to ink deals with the four leading payment networks indicates compromise on all sides, analysts say. For Isis, which originally aspired to build its own payment network — a plan that met with some resistance from merchants — the move indicates a reality check. For the credit card companies, the project highlights a growing realization that mobile payments are here to stay.

“This is as much an insurance bet for these credit card associations,” Egan said. “After they got over the threat over what this was going to do to traditional payment methods, it was a matter of how they should engage.”

In the race to be a part of the first digital wallet, credit card companies aren’t promising exclusivity, either. Sprint, the odd man out in the Isis venture, is working with MasterCard, Citi and First Data on the Google Wallet, an app for Android smart phones. Sprint also announced this week a joint initiative with American Express for a separate payment platform.

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