Sen. Barbara Boxer (D-Calif.), Chair of the Senate Ethics Committee, has paid some $30,000 since 2004 for the endorsement of embattled Rep. Maxine Waters (D-Calif.) in the context of a scheme that critics charge is unethical and amounts to Waters using her political station to benefit her family members.

According to the Washington Times, Waters “has turned political endorsements into a family business, using federal election laws to charge California candidates and political causes to include their names as her personal picks on a sample ballot, or ‘slate mailer,’ she sends to as many as 200,000 South Central Los Angeles voters.”

The slate mailer business, it turns out, is run by Waters’ daughter, Karen, via her public relations firm. Records show that Karen Waters’ firm has been paid more than $350,000 since 2004, and has billed a further $82,000 since California’s June primary, for its services in this regard.

It is a scheme that has been criticized by good governance groups including the Sunlight Foundation and Citizens for Responsibility and Ethics in Washington (CREW).

The Sunlight Foundation, in a blog post last month, equated the scheme to Waters “selling” her endorsement and noted that the amounts of money being paid to Waters’ own campaign committee in exchange for her endorsement often exceed the federal limit applicable to campaign donations.

Citizens for Responsibility and Ethics in Washington (CREW) meanwhile cited Waters as one of the most corrupt members of Congress in both 2005 and 2006 for operating the scheme. In both years, CREW noted that Waters’ “ethics issues stem from the exercise of [her] power to financially benefit her daughter, husband and son.” The 2005 rating placed Waters in the company of figures like former Reps. Randy “Duke” Cunningham and Bob Ney–both subsequently convicted of corruption-related offenses–and was directly connected to the operation of the political endorsement scheme.

Critics charge that Boxer has actively aided and abetted this scheme, both in the context of her 2004 and 2010 re-election campaigns.

In 2004, Boxer paid $25,000 for Waters’ endorsement. But ahead of this year’s California primary–in which Boxer faced no serious competition– and when it was well-known that Waters was under investigation by the House Ethics Committee, Boxer paid Waters $5,000 for her backing. It is this later payment that those monitoring Waters’ ethical woes say could act as an anchor tied to Boxer’s ankle, as she heads towards November.

“The amount is lower than what Boxer paid in 2004, yes,” said one Republican operative with whom we spoke. “But the Chair of the Senate Ethics Committee paying for an endorsement of an official under investigation for ethics infractions connected to her using her standing to benefit her family when that fact has been widely reported presents a major optics problem, to say the least. Ethics are already something of a surprise Achilles Heel for Boxer. Her committee let Chris Dodd off the hook, and she’s been criticized for using her repeat candidacies for the financial benefit of her family, too.” Added that operative, “This is an attack ad waiting to happen.”

Indeed, Boxer has been called out for steering about $500,000 in contributions made to her political action committee to her son’s consulting firm between 2001 and 2009. Like Waters’ scheme, this arrangement has attracted the attention of CREW, which focused on Boxer in a 2007 exposé entitled “Family Affair.” “Waters’ problems run far deeper, of course,” said the same operative. “But there are some interesting parallels here, to be sure.”

Boxer is facing what some observers say is her toughest political contest in a career that spans three decades, facing off against former Hewlett Packard CEO Carly Fiorina. The three-term Senator has previously said that she does not expect Democratic ethics scandals, including that of Waters, to affect the race.

Real Clear Politics’ polling average places Boxer about 4 points ahead of Fiorina, with recent SurveyUSA polling showing Fiorina ahead. The race has been rated a “toss-up” by numerous pollsters and analysts.