When McCain turned down a taxpayer-funded $5.8 million, his campaign was able to skirt spending limits that would have been in place until the Republican Party convention.
The issue in question is a $4 million line of credit obtained by the McCain campaign last year, which the DNC has argued was a violation of federal regulations. The McCain campaign and its bankers have denied that claim.
The lawsuit is a follow-up to a complaint the DNC filed with the FEC in February, which was not acted upon, since four FEC nominees were waiting Senate confirmation and the body lacked a quorum.
*Update: Later on Tuesday, RNC chief counsel Sean Cairncross called the lawsuit "meritless," the AP reports. Cairncross said that McCain was entitled to withdraw from the public finance system because he didn't take public funds during the primary season.