Senator John McCain’s presidential campaign on Friday stepped up its efforts to tie Senator Barack Obama to a community organizing group that has been accused of involvement in problematic voter registrations in several hotly contested states, including Colorado, Indiana, Nevada and North Carolina.

The group, the Association of Community Organizations for Reform Now, or Acorn, has long been a favorite target of conservatives. It made news this year when it was revealed that a brother of Acorn’s founder had embezzled almost $1 million from the organization but that Acorn had failed to disclose the theft for eight years.

“Barack Obama has made very inconsistent remarks about what his relationship with this organization is,” Rick Davis, Mr. McCain’s campaign manager, said in a conference call.

Mr. Davis said Mr. Obama had worked as Acorn’s lawyer and conducted training events for its leaders. He also noted a payment the Obama campaign made in February to an Acorn affiliate, Citizens Services Inc.