Union-backed critics of Wal-Mart Stores Inc plan to start their largest ad campaign in a bid to criticize the company during the holiday shopping season, the Wall Street Journal reported Monday in its online edition.

The campaign, which starts on Monday with radio spots in 40 markets, is an attempt to tap into negative publicity surrounding Chinese imports to Wal-Mart, the Journal said.

The ads note that Wal-Mart gets 70 percent of its merchandise from China, a figure the retailer disputes, the Journal said.

According to the Journal, the ads are the product of WakeUpWalMart.com, which is funded by the United Food and Commercial Workers Union.

The newspaper also said the radio spots will run through this Friday, which is traditionally one of the busiest shopping days of the year. The ads will run mostly in the South and Midwest where Wal-Mart has a strong presence, the Journal said, adding that television ads will appear in December.

Wal-Mart objected to the notion it was selling unsafe items, the Journal said.

Our commitment to low prices is never at the cost of safety. Product safety has always been and will continue to be a top priority, spokeswoman Sharon Weber said, according to the Journal.

(Reporting by Justin Grant)