BROOKE GLADSTONE: This is On the Media. I'm Brooke Gladstone.

BOB GARFIELD: And I'm Bob Garfield. This July, Mexican voters will go to the polls to elect a new president. Six years ago, they broke decades of one-party rule by electing President Vicente Fox. Under Fox, the government has loosened its longstanding stranglehold on the media, especially at election time. But this year, Mexican voters have experienced a new phenomenon - negative advertising. It's a tactic that reflects the influence of political consultants who come from north of the border. From Mexico City, Collin Campbell reports.

COLLIN CAMPBELL: This election season, three candidates are dominating the race for president. Roberto Madrazo has struggled in the polls, though his party held the presidency in Mexico for 71 years. Felipe Calderon is the candidate of the PAN, the party that made history by putting former Coca-Cola executive Vicente Fox in office in 2000, and Andres Manuel Lopez Obrador, the former leftist mayor of Mexico City, who until recently held a big lead in the polls. Two debates are being held during the campaign, and their format and content speak volumes about this evolving democracy. [SPANISH BROADCAST UP AND UNDER] In the first so-called debate, the candidates stood at podiums in a TV studio with no audience. The moderator introduced a series of themes given to the candidates in advance. They had about two minutes to speak on each theme. There were no questions, no journalists and little chance for the candidates to respond to each other. Even by the standards of U.S. presidential politics, this didn't qualify as a debate. It's a remnant of a corrupt one-party system that routinely squelched national debate, but overall, this year's campaign is offering a robust exchange of ideas. And behind that exchange, many political analysts here see the hand of U.S. political consultants. Media on both sides of the border cite rumors of everyone from Dick Morris to James Carville working for Mexican candidates. However, those mentioned either deny the reports or won't answer questions about them. Mexico's biggest step toward democracy took place during the last presidential election in 2000, when Vicente Fox prevailed over Francisco Labastida, a member of the party that ruled Mexico since 1929. Behind Fox's win, Rob Allyn, a Dallas-based political consultant who made a name for himself helping George W. Bush become governor of Texas in designing a potent attack ad against John McCain in the 2000 G.O.P primaries. Rob Allyn.

ROB ALLYN: I think consultants who work in the largest democracies in the world have the ability and in some ways the responsibility to carry values of democracy in other countries.

COLLIN CAMPBELL: As part of a larger campaign team, Allyn and his staff helped shape Fox's message, coached him through TV appearances, created ads, and even chose his ties. His opponent, Labastida, was also guided by a team of U.S. consultants, including James Carville. But Allyn successfully concealed his work from almost everyone, checking into hotels under false names, burning documents and avoiding the press.

ROB ALLYN: We did keep it quiet because, you know, I think if you're really putting the interests of your client first, then you shouldn't be out talking about the work that you do while it's still going on. It's not good for the consultant to become the issue. We're really not actors in this. We're just sort of moving scenery around on the set.

COLLIN CAMPBELL: And maybe not just moving around scenery. Many media watchers here say U.S. consultants may also be writing the scripts. [SPANISH AD]

COLLIN CAMPBELL: This is one of Mexico's first attack ads, which blanketed the airwaves in March. It shows Venezuelan hardliner Hugo Chavez threatening President Fox during a diplomatic spat and then cuts to leftist candidate Lopez Obrador telling Fox to shut up. Produced by Calderon's National Action Party, the ad drew on fears that Obrador could be an autocratic ruler in the Chavez mold. [SPANISH VOICES UP AND UNDER]

COLLIN CAMPBELL: Prior to that ad campaign, Obrador had won frontrunner status with retail politicking, traveling from one village to another with a populist message and a solid record as Mexico City mayor. But after the ads, polls showed a 20-point shift towards Calderon, and Lopez Obrador lost his lead. Pollster Dan Lund has worked for Obrador in the past.

DAN LUND: There's been dirty tricks and there's been mean things said about each other in this country, but nobody's ever experienced the full flavor of a U.S. negative-type campaign. In order to resist that, any campaign consultant worth his or her salt in the United States would say you need as much or preferably more money than the attacker, and you need a full-scale electronic media response.

COLLIN CAMPBELL: Lacking that response, Lopez Obrador was swept under. Pollster Francisco Abundis saw the impact play out in focus groups among once staunch Obrador supporters.

FRANCISCO ABUNDIS: You can see voters in pain, which is very interesting, and it's not very common. It's, “Please give me an argument.” You can see some type of anguish, and that's something that you don't see very often.

COLLIN CAMPBELL: The anguish was caused in part by misleading messages. One ad exaggerated the debt Lopez Obrador ran up as Mexico City mayor by a factor of five. Eventually, federal regulators forced Calderon's party to pull two of the spots. Many media watchers here decry the ads as the direct and destructive handiwork of U.S. consultants. But Rob Allyn, who declines to say whether he is participating in the current election, defends negative advertising as a hallmark of democracy.

ROB ALLYN: Negative advertising is never popular. It's not popular here in the United States. And so we shouldn't be surprised that it's no more popular in Latin America or Asia or anyplace else where it's been successfully used. It does work, in large part, because it opens up the debate and allows people to debate back and forth the positives and the negatives of each candidate and each party.

COLLIN CAMPBELL: But historians say the stakes are much higher than a few unpopular ads. Mexico has a history of violence and even murder during election season. That's one reason, many say, why the one-party system emerged and held power for so long. Sergio Aguayo, a political science professor at the Colegio de Mexico, says he's not sure Mexico's democratic institutions are strong enough to withstand the polarization wrought by U.S.-style negative campaigns.

SERGIO AGUAYO: Mexico has incorporated the worst of American politics, dirty politics. That's not the kind of contribution that one could respect.

COLLIN CAMPBELL: To political scientist and columnist Denise Dresser, this election will mark a new era in Mexican politics, the moment when candidates and voters witness the political power of TV advertising unleashed.

DENISE DRESSER: And that's going to be the real question of this election. Is it still possible to win doing old-style politics of reaching out directly to the people and using party machineries rather than using the media?

COLLIN CAMPBELL: If television is becoming key to winning the presidency here, then one-party rule may have been brushed aside only to be replaced by a hegemony of ad dollars. Voters will see more attack ads and they will feel the side effects, more cynicism and less participation at the polls. So as Mexico moves forward into a more democratic future, it may find itself exchanging its political problems for some of ours. For On the Media, I'm Collin Campbell in Mexico City.