Two high-profile Democrat leaders are on Tuesday evening due to lend their support to a union-backed campaign against Wal-Mart, the largest US retailer, in an indication of the political pressure the company is likely to face in the run-up to the 2008 presidential elections.

Senator Barack Obama of Illinois and John Edwards, the former senator from North Carolina who was the party’s candidate for vice-president in 2004, will participate in a call organised by the Wake Up Wal-Mart campaign aimed at mobilising protests against the retailer in the holiday shopping period.