Washington - Depending on your political persuasion, Barack Obama either made an innocent remark or a sexist slur when he referred to John McCain's message of change as putting "lipstick on a pig" during a US election rally.

The campaign of Republican nominee McCain slammed the comment - a common expression in the United States - as clearly directed at his running mate, Alaska Governor Sarah Palin. A web advertisement released Wednesday charged Obama with using a demeaning smear.

"Ready to lead? No. Ready to smear? Yes," the McCain ad said of his Democratic rival for the White House.

It was the latest accusation of sexism in the hotly-fought election campaign and dominated a cycle of US media coverage since Tuesday evening.

Obama on Wednesday said he meant nothing by it and derided the reaction as "phony outrage."

Palin is bidding to become the first female vice president in US history, and the complaints drew parallels to those made by supporters of Hillary Clinton's failed bid for the Democratic presidential nomination earlier this year.

The latest scandal began Tuesday, when Obama said McCain's claim that he would bring change to Washington rang hollow because of his support of President George W Bush's economic and foreign policy.

"That's not change, that's just calling ... the same thing something different," Obama said at a southern Virginia rally. "You can put lipstick on a pig, it's still a pig. You can wrap an old fish in a piece of paper called change. It's still gonna stink."

There was no specific reference to Palin, but his remarks came one week after the governor's speech accepting the Democratic nomination. One of her most memorable lines in that address: "They say the difference between a (Alaska) hockey mom and a pitbull: lipstick."

Jane Swift, a former Republican governor, said Obama owed Palin an apology, in the first conference call of the "Palin Truth Squad," a new network created by McCain's camp to push back against "Internet and liberal smears" since Palin was tapped as vice president in late August.

The network mirrors Obama's own truth squad set up earlier this year to counter false internet rumours, including that Obama, who is vying to become the first African-American president, is of the Muslim faith.

But the "lipstick" flap had Obama's camp in full damage control. The campaign e-mailed reporters a series of video clips and quotes of McCain and other Republicans using the same "lipstick on a pig" expression over the past two years. McCain used it earlier this year in reference to Clinton's health care plan.

Conservative and leftwing news commentators traded barbs over Obama's intentions on air Tuesday night.

Sean Hannity of Fox News called it part of a deliberate "strategy" to demean Palin. Mark Halperin of Time Magazine declared Tuesday one of the "lowest" days for media coverage in the 2008 election.

Obama on Wednesday blasted the McCain camp for "the latest made-up controversy" that distracted from important issues in the US election campaign.

"It would be funny, except of course the news media decided that was the main story yesterday," Obama said ahead of a discussion on education in Norfolk, Virginia. "The McCain campaign would rather have the story about phony and foolish diversions than about the future."