The alleged acts ACORN has been engaging in is voter registration fraud, not voter fraud.

There is a significant difference. Registration fraud does not necessarily lead to fraudulent voting. It appears that the cause of the voter registration fraud is an attempt by some Acorn workers who are paid for the signatures they get in registering voters. The more names they register the more they get paid.

McCain's ties to Acorn are no less, and probably more recent than Obama's.
McCain attended the Acorn convention in 2006 and was quite complimentary about their efforts.

The political spin about Obama and Acorn would seem to be tied to the predominance of poor, often minority individuals working and being registered by Acorn, a group that tends to vote for democrats. He has no more responsibility for what is taking place than McCain.