As a fluent public speaker, independent-minded wife, devoted mother and professional woman, Michelle Obama has been hailed as an invaluable asset to her husband Barack's mission to capture the Democratic 2008 presidential nomination.

Yet, while her style and performance are winning plaudits on the campaign trail, a little-reported business interest of Mrs Obama's has opened her husband up to one of the criticisms that politicians fear most - the taint of hypocrisy.

The apparent contradiction between Sen Obama's political calculation to join the Wal-Mart-bashing lobby, and his wife's profitable role with a company that makes money from Wal-Mart, is being closely scrutinised by "opposition" research teams working for rival White House candidates, The Sunday Telegraph has learnt.

They are collecting information about Mrs Obama's Treehouse ties, anticipating that - in a country where "going dirty" is a political way of life - the link may provide valuable ammunition in the election campaign.