Barack Obama never organized with or worked for ACORN. In Chicago he was a community organizer for a completely different organization, as described in detail in his memoir.
In 2008:
Because of Obama's record of working for positive change that helps working families, ACORN Votes, a federal PAC made up of ACORN leaders, endorsed Obama's candidacy for President. However, ACORN's successful effort to help 1.3 million people apply to become registered voters was a non-partisan effort to engage underrepresented voters and bring them into our democracy. Our voter registration organizers don't campaign for anyone as we help them complete registration applications.