Add Newark Mayor Cory Booker to the list of people who speak for the presidential candidates, and then veer wildly off message.

Booker on Sunday criticized President Barack Obama's campaign for attacking Republican Mitt Romney's tenure at a private equity firm. The Democrat and Obama supporter told NBC News he found the attacks "nauseating."

Booker's comment illustrated the difficulty candidates face controlling their message. It also raised questions about whether campaigns should be held responsible for what their supporters say or do.

Republicans have insisted that Obama repudiate comments by talk-show host Bill Maher and Democratic strategist Hilary Rosen about Romney's wife, Ann.

Romney's had his own problems with so-called "surrogates." His campaign distanced itself from Ted Nugent after the rocker made provocative comments about Obama.

(Copyright 2012 by The Associated Press.  All Rights Reserved.)