Sanders penned the book endorsement for "Buyer's Remorse: How Obama let Progressives Down" by Bill Press. "Bill Press makes the case ... Read this book," Sanders writes in a blurb featured atop the book's front cover.
"Bill Press makes the case why, long after taking the oath of office, the next president of the United States must keep rallying the people who elected him or her on behalf of progressive causes. That is the only way real change will happen," read Sanders' full quote. "Read this book."
On Saturday, Michael Briggs, Sanders' spokesman, stood by the comment.
"What Senator Sanders said in that book blurb is absolutely true," Briggs said. "The next president must do everything possible to reinvigorate American democracy and get working people to stand up for their interests. This country, under the outstanding leadership of President Obama and Vice President Biden, has come a very long way since President Bush left office."
Briggs added, "The next president can try to achieve bold proposals because of the foundation they put in place. Obviously, telling someone to read a book doesn't mean you agree with everything that's in the book."
Press' book, which is out a day after the Iowa caucuses on February 2, is about "the many ways President Obama has failed to live up to either his promises or his progressive potential," according to promotional materials.
Sanders is currently locked in a tight race to win the Iowa caucuses on February 1. A Des Moines Register/Bloomberg Politics poll
out Saturday found Sanders slightly trailing Hillary Clinton, 45% to 42%, but close enough to be within the poll's 4-point margin of error.
Clinton and Sanders have been fighting over Obama's legacy
, with Sanders laying claim more to the President's historic 2008 campaign and Clinton staking out his White House achievements.
Sanders met with Obama earlier this week and is clearly angling -- as is Clinton -- for an Obama endorsement.
A recent Sanders ad features a rising sun on the horizon, reminiscent of Obama's 2008 logo. He has talked about change coming from the bottom up, echoing a favorite Obama phrase. His slogan, "A Future to Believe In," is clearly borrowed from Obama's "Change We Can Believe In" motto.
In Iowa, Sanders has encouraged his supporters to see that same connection.
"It really reminds me very much of what happened here eight years ago. Remember that?" he said in Clinton, Iowa recently. "Eight years ago, Obama was being attacked for being pie in the sky, he did not have the experience. People of Iowa saw through those attacks then and they're going to see through those attacks again."