Congress and the attorney general are investigating a charitable foundation run by Marta Sahagun, President Vicente Fox's wife. The energy minister resigned in a spat connected to the first lady's political aspirations. And the presidential chief of staff quit this week, complaining about Sahagun in a blistering resignation letter.

The president's wife, referred to popularly as Martita, has become the focus of a raging debate in Mexican politics. The question of whether she will run to succeed her husband in 2006 dominates headlines, talk shows and comedy clubs. But underlying the gossip and chatter are serious allegations that Sahagun's political ambitions are damaging her husband's presidency.

"Every day she seems to be doing something else to kill the president's term," said political columnist Sergio Sarmiento. "She has become the greatest liability President Fox has."

Since Fox married Sahagun, his former press secretary, in a surprise ceremony at the presidential residence three years ago, the stylish first lady has exercised enormous influence. She abandoned the old-style decorative image of Mexican first ladies for a more high-profile adviser's role -- a kind of presidential partner Mexico has never seen. Some say she is the only person Fox fully trusts.

Sahagun frequently crisscrosses the country, appearing at schools, hospitals and orphanages, giving out bicycles and other items from Vamos Mexico -- Let's Go Mexico -- her charitable foundation that raises about $1 million a month. A federal investigation is underway to determine whether money from the national lottery was improperly funneled to the foundation or its private charities.

But Sahagun remains popular with the Mexican public. She has been coy about her political ambitions, telling a newspaper this year, "You will have Marta around for a long time. I think Mexico is now ready to have a presidenta," a female president.

On Tuesday, Fox emphatically ruled out a presidential run by Sahagun; he said that when his term ends, the couple would retire to their ranch to write, ride horses and be with their children. On Wednesday in Brazil, Fox issued a statement responding to a "supposed crisis" in his government: "The future of the country is in good hands, in responsible hands, in professional hands . . . The president of Mexico is very respected, known for seriousness, for professionalism."

As Fox was speaking out publicly to cool the controversy, Sahagun had no comment. Opinion polls consistently show her running second only to Mexico City Mayor Andres Manuel Lopez Obrador as a presidential hopeful.

Many political analysts regard Sahagun as a key reason that Fox's presidency is sputtering. Day after day in newspapers and on television, critics say Sahagun's influence and visibility get in the way of Fox's agenda, contributing to the resignations of top aides, putting the president on the defensive and creating deep divisions within his National Action Party, or PAN.

"There is not a day, not an hour, not a minute in which Martita is not meddling in the country's affairs and messing them up even more," columnist Guadalupe Loaeza wrote in the Reforma newspaper.

"She has been a totally unnecessary distraction for him," said political analyst Gabriel Guerra. "Even his most optimistic enemies could not have envisioned this."

Fox's falling fortunes were evident in important elections on Sunday, when the PAN candidates lost in governor's races in Chihuahua, Durango and Zacatecas states. But perhaps the biggest blow came Monday, when Fox's chief of staff, Alfonso Durazo, resigned in a rare public split with the man he had served for four years.

In a 19-page letter, Durazo said he had become disillusioned with Fox's administration and how it has been influenced by Sahagun's political ambitions. Durazo accused Fox of behaving as if he were a member of the Institutional Revolutionary Party, or PRI, which Fox unseated in 2000 after 71 years of authoritarian rule, because PRI presidents handpicked their successors.

"The desire for a government to decide who the next president will be or won't be was the original sin of the old regime," Durazo wrote, adding, "The country has certainly advanced politically, enough that it is ready for a woman to reach the presidency of the republic. Nonetheless, it is not prepared to have the president leave the presidency to his wife."

Fox blasted Durazo in comments to reporters Tuesday: "I placed my confidence in him for four years, and the result was this: He gave out confidential information, private information, and now he has made incorrect interpretations." Fox accused Durazo of "taking out his crystal ball and constructing these stories."

But Guerra, the political analyst and a spokesman for two previous presidents, said Durazo's letter "brings to the headlines what people have been whispering about for months. You're starting to see the toll of Marta."

Guerra said Durazo's letter was a rare look at the frustration that has contributed to the departures of a number of Fox administration officials, including Energy Minister Felipe Calderon, a leading National Action Party member. Calderon quit in May after Fox publicly scolded him for being indiscreet about his presidential aspirations. His supporters in the party complained that Fox tolerated similar behavior from Sahagun.

"The problem we have to resolve is whether it is possible for the wife of the president to run for president," Calderon said in an interview. "There are no rules about it, no precedent." Calderon declined to comment on his feelings about Sahagun and his resignation, saying the matter should be discussed privately by Fox and the PAN leadership "to avoid more damage to the party and the president."

Calderon, Interior Secretary Santiago Creel and other PAN members have been mentioned as possible candidates in 2006, but none comes close to Sahagun in popularity polls. Analysts said the perception that Sahagun's flirtation with running is hurting other PAN candidates has created resentment and division within party ranks.

Vamos Mexico has caused the worst headaches for Sahagun since January, when the Financial Times reported that the foundation had spent 34 percent of its income on overhead. The newspaper also reported that the foundation had donated only 30 percent of its income to charitable causes in its first 15 months of existence, including only 10 percent of the nearly $7 million raised at its kickoff event, a $10,000-a-ticket concert by Elton John.

Sahagun responded in an emotional speech that the article was filled with "lies and calumny." Last month Vamos Mexico released audited financial statements through 2003 showing that just under half of the $27 million it had raised since its launch had been given to charities. The foundation invited the federal attorney general's office to review its finances.

Congress has also launched an investigation into Vamos Mexico. Some opposition legislators have alleged Sahagun improperly steered money from Mexico's national lottery to private projects she is funding through the foundation to further her political ambitions.

Guadalupe Mendez de Garcia, a friend of Sahagun's since elementary school, said the criticism does not bother the first lady. "She has had great satisfaction and great achievements," she said. "There has been severe criticism, but she is a person who goes ahead with optimism . . . she is a very brave woman."

Asked if Sahagun would run for president, Mendez said, "I wouldn't dare answer that question. The only thing she has done is to serve Mexico. She works for the good of Mexico, for the good of women. And nobody is going to stop her."

Mexican President Vicente Fox and his wife, Marta Sahagun, criticized for her ambition, give separate interviews to TV crews at Mexican Consulate in Chicago.