CHICAGO – Republican U.S. Rep. Joe Walsh and Democratic challenger Tammy Duckworth traded accusations over campaign finances this week as one of the most closely watched congressional races nationwide grew even more contentious.

Duckworth, a former official in President Barack Obama's administration, claimed that Walsh has close links to a group that is a top contributor to a super PAC supporting the tea party candidate's re-election. Legally, super PACs and candidates they support can't be in direct contact.

Duckworth campaign officials said Wednesday that they planned to file a complaint with the Federal Election Commission alleging illegal coordination. Walsh's campaign has denied involvement in the past decade with the group, Americans for Limited Government.