WASHINGTON — The top-tier presidential candidates have some personal finance numbers in common — six- or seven-figure book deals.

Writing a book has become a prerequisite to running for president — a means to explain views in depth, to set the record straight, and to add a bit of gravitas. But while nearly all the candidates put pen to paper, it is mainly those ranked high in the polls who make any real money out of it.

Senator Clinton made an $8 million book deal for “Living History,” published in 2003. In the last two years, the New York senator chalked up about $1.2 million in book royalties.

Though his campaign may be in a financial rut, Senator McCain’s book sales are consistently profitable. The Republican from Arizona, whose campaign has suffered recent setbacks, made $80,390 in 2006 from Random House book royalties and about $255,000 in book profits the year before.

Also finding literary success was Senator Obama, a Democrat of Illinois, who received a $425,000 advance against royalties for “The Audacity of Hope,” published last October. Listed as a best-seller for dozens of weeks, the book has sold 1 million copies, according to Nielsen BookScan, which tracks about 70% of book sales.

HarperCollins paid a former Democratic senator of North Carolina, John Edwards, a $500,000 advance for “Home: The Blueprints of Our Lives,” a collection of mini-memoirs he edited.

Mr. Edwards donated his royalties to charity — something politicians sometimes do with a portion of their book proceeds.

And big book profits are not a reality for many White House hopefuls, even if they’re well-known. “I think it’s a losing proposition for most candidates in that they’re not looking at those books for instant wealth,” said publisher Jonathan Karp of Twelve, an imprint of the Hachette Book Group that released Mr. McCain’s “Hard Call” this month. “Because most of them are not natural writers or natural storytellers. Most of them fall back on boilerplate and cliché.”

Mr. Karp says Senator Kerry’s “A Call to Service: My Vision for a Better America,” published in October 2003, was considered a snooze. The 2004 Democrat nominee made about $89,000 in book royalties during his run-up to the election in 2003, but didn’t report any for 2004. Compare that with Mrs. Clinton’s 2004 royalties totaling about $2.3 million for “Living History.”

A used copy of Mr. Kerry’s book can now be found on Amazon.com for 1 cent.

Although he leads the GOP field in most polls in the early voting states of Iowa and New Hampshire, Mitt Romney’s “Turnaround,” published in 2004, has sold only about 11,000 copies.

The less prominent candidates in the 2008 race may sympathize. Governor Richardson’s memoir “Between Worlds: The Making of an American Life,” published in November 2005, earned the New Mexico governor somewhere between $15,000 and $50,000 in book royalties last year. It sold about 11,000 copies in hardcover and paperback, according to BookScan.

If candidates can’t draw voters to a town hall meeting, they probably can’t get them to buy their books, said David Rosenthal, executive vice president and publisher at Simon & Schuster, which has published books by Mrs. Clinton and President Carter.

Rep. Tom Tancredo, a Republican of Colorado, received between $15,000 and $50,000 in royalties or “In Mortal Danger: The Battle for America’s Border and Security” in 2006. The book, published last summer, has sold 10,000 copies, according to BookScan.

Rep. Dennis Kucinich, a Democrat of Ohio, also a presidential candidate in 2004, made a mere $1,700 that year for “A Prayer for America,” published in late 2003. It has sold 8,000 copies so far, according to BookScan.

But not every candidate low in the polls is destined for literary obscurity. A former Arkansas governor, Mike Huckabee, who has written five books, made $148,750 in book royalties from Margaret McBride Literary Agency last year. The agency helped Mr. Huckabee, who gained national attention after he lost more than 100 pounds, release his weight-loss book “Quit Digging Your Grave with a Knife and Fork” in 2005.

Senator Biden, a Democrat of Delaware, seems to be another exception among less prominent candidates.

He received an $112,000 advance in 2005 for “Promises to Keep: On Life and Politics,” which was released last month. According to his financial disclosure forms, he was to receive another installment of $112,000 when he finished the book’s manuscript.

Compare Mr. Biden’s six-figure advances with those of Senator Dodd, a Democrat of Connecticut, who received a $30,000 advance for “Letters to Nuremberg,” expected to be released in September.

Timing also counts. Mayor Giuliani published “Leadership” about a year after September 11, 2001. It has sold 665,000 copies in hardcover and garnered the former New York mayor $146,092 in royalties last year.

Mrs. Clinton’s memoir, largely about her years as first lady, “was a hugely successful book,” said Mr. Rosenthal, who said it sold about 1.5 million copies. “Giuliani’s was a good seller because it followed 9/11.”