Since falling short last year in his bid for the U.S. Senate, the outspoken Stephen P. Laffey has been unusually quiet.

Now we know what the former Cranston mayor has been up to — Laffey has written a “tell-all” book about politics, his 2006 primary campaign against former Republican U.S. Sen. Lincoln Chafee and the future of the national Republican Party.

Laffey’s nonfiction book, Primary Mistake, will be published Sept. 13 by Sentinel, an imprint of publishing giant Penguin Group Inc. Sentinel publishes conservative-leaning books, such as the recent memoirs of former U.S. House Majority Leader Tom DeLay. Laffey expects to support the book with a tour and publicity campaign this fall.

In 2006, the national Republican Party backed Chafee — one of the more liberal Republicans in the Senate — over Laffey, a “Reagan conservative,” according to promotional material from Penguin. The GOP’s larger strategic thinking was that only Chafee could hold the seat in deep blue Rhode Island, especially in the face of a national tide swinging against Republicans.

Laffey takes issue with the strategy, as suggested by the subtitle of his book: How the Washington Republican Establishment Lost Everything in 2006 (and Sabotaged My Senatorial Campaign).

Laffey will argue in the book that his race epitomized what went wrong with the Republican Party, which lost control of the lishment Lost Everything in 2006 (and Sab otaged My Senatorial Campaign).

Laffey will argue in the book that his race epitomized what went wrong with the Republican Party, which lost control of the House and Senate in the last election. “The national Republican Party lost power because it put power in front of principle,” Laffey said yesterday. “I wanted to set forth some principles that we should hold on to.”

His editor at Penguin Group, Bernadette Malone Serton, said that Laffey tells the story of the campaign with stunning candor.

“Steve Laffey is so candid in talking about what Washington Republicans did to him that the rest of the country needs to know why they lost the Senate in 2006,” she said.

“And he names names in his book,” she promised.

Neither Laffey nor Serton would describe the contents of the book in detail before it is published, but both said it has nationwide implications.

“It’s a wake-up call to all Republicans for 2008,” Serton said, “because if these are the kind of decisions [by Republican leaders] and the games that are going to be played, that could very well affect the outcome of the presidential election.”

Laffey, who is traveling out of the country, explained by telephone yesterday why he wrote the book.

“I’m a very future-oriented person,” he said. “I don’t sit around and stew. I thought immediately that my race had a lot of implications nationally. I thought I had something to offer nationally for the party and the public.

“I really thought my race was the epitome of how the national Republican Party lost power and did the wrong thing over the last six years.”

The book ends with “a very positive message for the future, a very hopeful” message, he said.

Primary Mistake is Laffey’s first book. He admits that when he started, he had no idea how to write one. He joked, “I bought all these books — you know: ‘How to get a book published for dummies.’ ”

The project took about four months. Few people knew he was working on a book. He credits a few close advisers for their help, including his former campaign spokeswoman, Nachama Soloveichik.

“Writing a book was the most disciplined thing I’ve ever had to do,” Laffey said. “I have the utmost respect for anybody who’s written a book. Page after page can flow out onto paper, and then there will be a paragraph that you argue about for a week.

“It’s a journey to write a book.”