Adolfo Aguilar Zinser, an outspoken intellectual who was one of the chief architects of the historic 2000 election of President Vicente Fox, died Sunday in a traffic accident near the resort town of Tepoztlan, where he had a home. He was 55.

Aguilar Zinser was killed when the Jeep Liberty he was driving was involved in a head-on collision with a bus, according to Alejandro Retana Pina, a spokesman for the attorney general's office in the state of Morelos, where the accident occurred. Retana said a car behind Aguilar Zinser's was also involved in the crash and that four people were hospitalized.

"His contribution to Mexico was enormously significant," said former foreign minister Jorge G. Castaneda, another left-leaning academic who worked for years against Mexico's authoritarian governments of the past and also played a key role in the Fox election. "It was in different trenches at different times and with different people, but always with the same purpose -- to bring some kind of representative democracy to Mexico."

Fox had no immediate public comment on Aguilar Zinser's death.

Although Aguilar Zinser was at Fox's side nearly constantly during the raucous campaign, then served initially as his national security adviser, the two had a severe falling out over Aguilar Zinser's conduct after Fox appointed him as Mexico's ambassador to the United Nations.

At the United Nations during the run-up to the invasion of Iraq, Aguilar Zinser was a vocal critic of the Bush administration's desire to use military force. That caused a political nightmare for Fox, who was trying to balance his desire not to provoke Washington's ire with Mexican public opinion that overwhelmingly was against the use of U.S. military force in Iraq.

Aguilar Zinser was ultimately recalled as U.N. ambassador in November 2003 after he gave a speech in which he accused the United States of dismissively regarding Mexico as its "back yard," a comment that then-Secretary of State Colin L. Powell called "outrageous."

"I am an undiplomatic diplomat," he wrote in an angry, unapologetic and public resignation letter in which he accused Fox of betrayal and the United States of ignoring international law.

In the year and a half since, Aguilar Zinser had written a regular column in the Reforma newspaper and hosted a public affairs program on television.

A stylish lawyer who attended Harvard, Aguilar Zinser was born into a prominent Mexican family in 1949. His grandfather, Miguel Angel de Quevedo, was a noted ecologist, something of a Mexican Johnny Appleseed.

A fluent English speaker, Aguilar Zinser had taught at the University of California at Berkeley and was well known in Washington circles. As an avowed progressive with John Lennon-style glasses and a taste for Hermes ties, Aguilar Zinser at times seemed an odd sidekick to Fox, a pro-business rancher and former Coca-Cola executive who favors cowboy boots. As national security adviser, Aguilar Zinser was fond of saying the most pressing threats to Mexico's security were not kidnappers and terrorists, but poverty and environmental degradation.

But Fox and Aguilar Zinser shared a dream of bringing down the Institutional Revolutionary Party, or PRI, which had run Mexico since 1929. Aguilar Zinser had blasted the PRI in articles, speeches and as a member of both the Chamber of Deputies and the Senate.

"He loved to be where the action was," said Lorenzo Meyer, a historian who taught Aguilar Zinser in college. "He was so full of energy and enthusiasm."

Researcher Gabriela Martinez contributed to this story.

Adolfo Aguilar Zinser was Mexico's ambassador to the United Nations until he was recalled by President Vicente Fox after chiding the United States.