These days, you can’t run for president without writing a book.

Yes, Barack Obama’s The Audacity of Hope is the famous one, and it was actually pretty good. (Dreams From My Father doesn’t count—it’s a memoir and wasn’t released during a presidential campaign.)

But pretty much every other serious candidate has also written a book or two during election season, pitching  their ideas and policies with a bit of autobiography and a few anecdotes from the campaign trail.

They’re usually … not so good. (That’s what you get with a ghostwriter and a lot of time spent on a campaign bus.)

And that’s OK. These books are rhetorical devices. They’re not meant to be critically acclaimed. But given the potential personal rewards, the candidates would surely like them to be commercially successful.

Unfortunately, according to numbers collected by Nielson BookScan, which covers print sales at 85 percent of booksellers (though not e-books), that doesn’t look likely for most.

While the pattern looks simple—release the book, get a bunch of sales—it appears candidates who publish while they’re riding high in the polls get an additional boost. Ben Carson’s chart-topping sales in October were likely helped by his chart-topping polling, and Huckabee’s debut in January came near the height of his favorability ratings.

But sales trail off rather quickly. Huckabee’s good start had petered off to less than 1,000 copies a month by May.

And some never get off the ground. Marco Rubio never broke 3,000 copies a month. By comparison, The Girl On The Train, a thriller by Paula Hawkins, has sold 1.2 million copies since its release in January.

These figures include hardcover and paperback sales. This list only includes campaign books published by presidential candidates in 2015, so older books like Scott Walker’s Unintimidated aren’t included. Neither is Rand Paul’s Our Presidents & Their Prayers, which focuses on faith. No e-books either, so Jeb Bush’s Reply All wasn’t tracked.