Heard an interesting radio spot this morning. Wal-Mart is asking shoppers to take the "Wal-Mart Challenge": the retailer wants consumers to bring their receipt from Cub Foods, buy the same items at Wal-Mart, and compare prices.

You can see what I'm talkling about at wal-mart.com/minneapolis.

What's remarkable about Wal-Mart's campaign is that it has to do it in the first place. Just a few years ago, Wal-Mart was the undisputed price king of not just groceries but practically everything. The retailer didn't have to convince people it offered the lowest prices. Everyone just knew it.

Not anymore. Last year, WSL Strategic Retail released a report that said 86 percent of Wal-Mart shoppers no longer believe the retailer had the lowest prices. A Morgan Stanley survey was only slightly more generous to Wal-Mart: 60 percent of consumers think Wal-Mart no longer beats its competitors.

Consulting firm Kantar Retail also says Target, which has been heavily pushing its P-Fresh grocery format, has also significantly reduced the price gap between itself and Wal-Mart.

Interesting though that Wal-Mart focuses on Cub Foods and not Target in the radio ads. Perhaps Wal-Mart doesn't think Target represents much of a threat on groceries.

Yet.