Puerto Vallarta, Mexico - The race to pick Mexico’s next president took a historic turn last Sunday night, with the ruling party picking a woman, the first from a major party, as its candidate to hold off a strong push from the largest opposition party to reclaim the post it had held for more than seven decades.

The candidate, Josefina Vázquez Mota, triumphed over two others in the primary of the conservative National Action Party, including a former cabinet minister who was said to be the first choice of President Calderón, who by law cannot seek a second six-year term.

The stage is now set for the July 1st contest, which will be carried out in tumultuous times with a war against drug cartels that has cost more than 47,000 lives since 2006, worries that organized crime will interfere with the election, and signs the country’s relatively stable economy may be slowing.

The race is likely to be watched closely among policy makers in the United States for any sign of dialing back the deepening cooperation between the two countries in battling the drug cartels. So far no candidate has proposed lessening those ties, but all have promised, without many specifics, to reduce the violence without giving up the fight against drug gangs.

Ms. Vázquez Mota, 51, will compete with two other major party candidates selected in December. Enrique Peña Nieto, 45, the former governor of Mexico State, is the front-runner in polls and is trying to return his party, the centrist Institutional Revolutionary Party, known as the P.R.I., to power it lost in 2000 after ruling for most of the 20th century.

Andrés Manuel López Obrador, 59, representing the leftist Democratic Revolutionary Party, lost a narrow race to Mr. Calderón in 2006 and is his party’s standard-bearer again.

Ms. Vázquez Mota exulted in her victory, winning far more than the necessary votes to avoid a runoff and basking in the potential to become the first woman president here. “I am going to be the first woman president in history,’’ she said in a victory speech, using “Presidenta,” the feminine form of the word.

Large numbers of women could be seen at some polling places and many analysts, including those with ties to the P.R.I., have long said she would pose a serious challenge to Mr. Peña Nieto, who maintains a commanding lead.

“She has a good outreach to people outside the party,” said Andrew Selee, director of the Mexico Institute at the Woodrow Wilson Center in Washington. “She is likely to be able to give her party a new lease on life in the election.”

Women voters have been playing a bigger role in recent presidential elections, leading many pundits to speculate Mexico is poised to follow other Latin American countries, including Brazil, Argentina, Costa Rica, and others, in electing a woman.

Mr. Lopez Obrador, while trailing in polls, still has a passionate base of supporters from his last run and Mr. Peña Nieto has a wide, deep political network (the P.R.I. controls 20 of 31 governors posts and many mayors as well) to draw from. His party also has far and away the most money and he is expected to get further mileage from his telegenic qualities.

Mr. Peña Nieto was very gracious upon learning of Ms. Vásquez Mota's win in her party's primary. “Congratulations and welcome to the democratic contest," he said in a Twitter message. “May it be for the good of Mexico.”