A watchdog group filed an elections complaint Monday against Republican presidential candidate Newt Gingrich over his lucrative mailing list, which the campaign has said it purchased from the candidate for $42,000.

Citizens for Responsibility and Ethics in Washington (CREW) alleges in a complaint with the Federal Election Commission that the campaign’s payment to Gingrich for the mailing list amounts to an unlawful personal use of campaign funds. The group, which monitors government officials, also says Gingrich appears to be improperly commingling book sales and campaign events.

The mailing list payment, which was not declared on FEC disclosure forms, was first reported by The Washington Post.

“Newt Gingrich will do anything to make a buck, even sell his own mailing list to his campaign,” said CREW executive director Melanie Sloan. “He has a long history of playing fast and loose with ethics rules, so it should surprise no one to learn he is at it again.”

Gingrich campaign spokesman R.C. Hammond responded Monday that “if the FEC considers the complaint, they will find that the rules are being followed and published regulations are being enforced.”

Mailing lists are routinely bought and sold by political campaigns, but the Gingrich case is unusual because the campaign says it paid the candidate personally for the list. Most such lists are owned by a political committee or group rather than by the candidate themselves.

Gingrich declared in disclosure documents filed in July that he was owed $47,005 by the campaign for “direct mail list/travel.” Campaign spokesman R.C. Hammond told The Post that the campaign then paid Gingrich $42,000 for the mailing list during the third quarter, although the expense was not declared on FEC disclosure documents as required.

Hammond said that Gingrich himself — not any of the firms he previously headed — owned the list and that failing to note the payment had been an oversight.

But CREW notes in its complaint that the mailing list was not included as an asset in Gingrich’s financial disclosure records, which are filed by presidential candidates. Therefore, the group argues, the list appears to be owned by Gingrich Productions, which is the name of the holding firm now headed by Gingrich’s wife, Callista.

CREW also points to reports in The Post and the New York Times detailing how Gingrich Productions routinely sells books and other materials at the same venues where Gingrich holds campaign-related events.

The sale of the mailing list, as well as commingling of book sales and campaign activities, violate FEC prohibitions on corporate contributions to a political campaign, CREW alleges in its complaint.

Gingrich aides have said the campaign and the production company keep their finances and activities segregated as required and adhere to all FEC guidelines.

Federal law prohibits candidates from using campaign resources to profit personally or from using corporate funds to subsidize a campaign.