A Mark Penn memo reportedly said the U.S. wouldn't vote for a candidate who was not 'fundamentally' American.

(CNN) – John McCain's recent campaign commercial linking Barack Obama to vapid celebrities was unanimously criticized in Democratic quarters, but one of the party's leading strategists says it did the job.

In an op-ed in the Politico, former top strategist Mark Penn to Sen. Hillary Clinton said negative ads are often effective in forming public opinion around a candidate, and specifically pointed to the McCain campaign's recent ad featuring Paris Hilton and Britney Spears as an example of an effective television spot.

Related: Memo could keep Clinton-Obama rift open

"Fair or not, as advertising it did its job: It used humor, stuck viewers with memorable images and created a debate, just as Lyndon Johnson’s 1964 ‘Daisy’ ad, Walter Mondale’s ‘Red Phone’ spot 20 years later and Hillary Rodham Clinton’s ‘3 a.m.’ commercial in 2008 did," Penn wrote.

Watch: McCain's "Celeb" ad paints Obama as out-of-touch celebrity

Penn, who was behind Clinton's headline-grabbing "3 a.m." ad that questioned Obama's readiness to lead during a national security crisis, also said the Illinois senator should have responded more effectively to the Hilton/Spears ad.

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Mark Penn has resigned his post as chief strategist to the Clinton campaign.

(CNN) — Mark Penn has given up his role as chief strategist for the Clinton campaign, it was announced Sunday.

"After the events of the last few days, Mark Penn has asked to give up his role as Chief Strategist of the Clinton Campaign; Mark, and Penn, Schoen and Berland Associates, Inc. will continue to provide polling and advice to the campaign," Clinton Campaign Manager Maggie Williams said in a statement.

Sources in the Clinton campaign tell CNN's Mike Roselli Penn realized over the weekend that he needed to step aside, and that Clinton was disappointed that he had met with the Colombians.

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Earlier: Rendell criticizes Penn, hints he should be fired

Related: Penn calls trade meeting 'error in judgment'

Rendell, left, criticized Penn Sunday over his meeting with the Colombian government.

(CNN) — A key backer of Hillary Clinton's White House bid gave her top strategist Mark Penn a less than ringing endorsement Sunday, following news Penn had met with the Colombian ambassador to promote a free trade agreement the New York senator opposes.

Appearing on NBC's Meet the Press, Pennsylvania Gov. Ed Rendell didn't exactly give the longtime Clinton pollster a vote of confidence.

Asked directly if the campaign should cut ties with Penn, Rendell said, "Well there are a lot of issues in which you can raise that question."

"Yeah, I think you've got to make it very clear for someone who is a consultant, who you are representing and who you are not representing and I would hope that Mr. Penn when he talked to the Colombians made that clear," Rendell also said. "And it doesn't sound to me like he did and that's something the campaign should take into question."

Late last week it was reported by the Wall Street Journal that Penn had met with the Colombian ambassador on Monday to promote a free trade agreement that Hillary Clinton has sharply criticized on the campaign trail. Penn's P.R. firm Burson-Marsteller had a contract with Colombia to promote the agreement, though a spokesman for Colombia's president told the paper he didn't know if Penn was representing Clinton or his P.R. firm in the meeting.

On Friday, Penn said he was acting in his role as CEO of Burson-Marsteller and called the meeting a "error in judgment." Upset with that characterization, the Colombian government cut ties with Penn's firm on Saturday.

Mark Penn is a chief strategist for the Clinton campaign.

(CNN) — Clinton campaign strategist Mark Penn said Friday it was an "error in judgment" for him to meet with the Colombian ambassador to advocate for a free-trade agreement Hillary Clinton has said she opposes.

"The meeting was an error in judgment that will not be repeated and I am sorry for it," Penn said in an issued statement. "The senator's well known opposition to this trade deal is clear and was not discussed."

The meeting, first reported by the Wall Street Journal, occurred Monday. Penn appears to have been acting in his role as chief executive of the international communications and lobbying firm, Burson-Marsteller Worldwide. The Colombian Embassy hired the firm to help achieve congressional approval of a bill allowing free trade with the country — a proposal Clinton has sharply criticized.

Earlier Friday, Clinton spokesperson Mo Elleithee said the New York senator "remains steadfast against the Colombian Trade Bill," and maintained Penn's meeting was "not in any way done on behalf of the campaign."

Clinton has sharply criticized America's free trade agreements as she campaigns for the Democratic presidential nomination, and was particularly critical of rival Barack Obama in February when it was reported that one of his chief foreign policy advisors had suggested to a Canadian official that the Illinois senator was not as anti-free trade as he was claiming to be on the trail.

"I don't think people should come to Ohio and tell the people of Ohio one thing and then have your campaign tell a foreign government something else behind closed doors," Clinton said then. "That's the kind of difference between talk and action and that I've been pointing out in this campaign."

"I would ask you to look at that story, substitute my name for Senator Obama," she also said. "If some of my advisers had been having private meetings with foreign governments and basically saying ignore what I'm saying because it's only political rhetoric … I think it raises serious questions."

Earlier: Campaign: Penn wasn't representing Clinton in trade meeting