CHICAGO, Aug 25 (Reuters) - Walmart (WMT.N) said in a blog post on Wednesday its advertising arm will start offering brands a demand-side platform (DSP) that will help them buy digital ad space more efficiently, just months ahead of the holiday shopping season. A DSP is an automated tool that allows advertisers and agencies to buy, monitor and manage video, mobile and search ads in one place. Prior to DSPs, salespeople and ad buyers were responsible for purchasing and selling ads for companies. Walmart has been aggressively expanding its ad unit - recently rebranded as Walmart Connect - after years of stuttering progress, making better use of its wealth of shopper data to attract major advertisers. The world"s biggest retailer faces a huge task chasing Amazon"s multi-billion dollar ad business, while also battling smaller rivals including Target (TGT.N) and Kroger (KR.N) which are building their own ad platforms. Now, Walmart Connect is partnering with The Trade Desk (TTD.O), a prominent ad tech firm, to court more advertising revenue. Walmart touted its vast online, app and in-store shopping data as a means to help advertisers improve how their promotions and ads are targeted to potential customers, and effectively measure how those ads lead to sales. "Even though the COVID-19 pandemic accelerated the growth of e-commerce sales last year, more than 80% of the total retail sales in the U.S. are still happening offline," Rich Lehrfeld, senior vice president of Walmart Connect, said. "By connecting and measuring both online and in-store performance, the Walmart DSP can offer targeting, reporting, and omnichannel insights down to a granular level." The company - which has roughly 4,700 brick-and-mortar stores - said the platform would be available to some brands by the end of October.