Florida senator and presidential candidate Marco Rubio published his memoir, An American Son, in 2012, a year and a half into his first (and last) Senate term. The book—for which Sentinel, Penguin’s conservative imprint—paid an $800,000 advance, didn’t set the world on fire, but at least it outsold his next book, American Dreams: Restoring Economic Opportunity for Everyone, which was published earlier this year.

There’s a lesson here: if you’re planning on running for president, don’t publish two books in a row with the word “American” in the title.

And another lesson: if you’re a candidate known for your money problems, don’t use your Political Action Committee to pay your ghostwriter, unless you enjoy the frisson of ethics violations.

On Sunday, the National Journal’s S.V. Dáte reported that there are some discrepancies over the $20,000 paid to Rubio’s ghostwriter, the political strategist Mark Salter, in 2012. Salter says that he was paid for “projects unrelated to the book.” But according to a 2013 Tampa Bay Times story, the payment was “for help writing a memoir.”

Why is this significant? Dáte explains:

If Rubio used leadership PAC money to help produce a book that put money in his own pocket, he may have violated Senate Ethics rules—not something a presidential candidate in the heat of primary season wants to be dealing with.

Dáte hasn’t heard back from Rubio’s spokesman about the $20,000, but as he points out, the irregularity isn’t necessarily illegal—“there appears to be no election-law prohibition against using ‘leadership PAC’ money in ways that personally benefit candidates.” Rather, the issue is Senate Rule XXXVIII, paragraph 2:

No contribution . . . shall be converted to the personal use of any Member or any former Member. For the purposes of this rule “personal use” does not include reimbursement of expenses incurred by a Member in connection with his official duties.

In other words, if it turns out that a contribution by Rubio’s PAC led to the publication of a book that enriched him, we may be dealing with “personal use.” Which, in turn, means that Rubio’s rivals will be able to start talking about the candidate’s money problems again.

This controversy may very well reignite the sales of An American Son, but as Jonah Lehrer can attest, book sales aren’t always worth the trouble.