MEXICO CITY (AP) _ Luis Donaldo Colosio, assassinated on Wednesday while campaigning for the presidency, was usually shy and tense until he mounted a platform and started to speak to supporters - usually poor people.

Then the words would start to flow easily, he would relax, and he related to his listeners.

It was on such a campaign rally at a poor neighborhood in Tijuana, across the border from San Diego, that Colosio - the front-running nominee of the ruling party - was promising better community services when he was shot.

The attack stunned Mexico, already rattled by a New Year's Indian uprising in southern Chiapas state, and came amid a growing movement to clean up the country's election system.

Since departing President Carlos Salinas de Gortari named him the party's candidate on Nov. 28, the 44-year-old Colosio himself had insisted he was all for a clean election.

But skepticism was widespread.

Colosio managed Salinas' 1988 campaign, guiding him to a narrow victory in an election foes decried as tainted.

People from all levels have demanded there be no repeat in the Aug. 21 balloting of the shenanigans that permitted Colosio's Institutional Revolutionary Party, or PRI, to keep power through the 65 years since it was founded.

Colosio's main rival, Cuauhtemoc Cardenas of the left-leaning Democratic Revolutionary Party, announced Wednesday on hearing news of the shooting that he was halting campaigning until further notice.

Colosio's other, undeclared rival, Manuel Camacho Solis, called the shooting ''a great offense against us all.'' Camacho had made no bones about his disappointment at being passed over by Salinas for the nomination, but had declared just Tuesday that he would not seek the presidency.

Salinas had named Camacho, a former Mexico City mayor, to be foreign minister, then government peace envoy to the rebels.

Many PRI elders considered Colosio's humble beginnings would make him the ideal candidate in an open electoral process.

Colosio was born Feb. 10, 1950, in Magdalena de Kino, a small town in Sonora. His father, Luis Colosio Fernandez, was a rancher and minor meat- packing house operator.

Colosio was poor throughout his childhood, and relied on scholarships and similar help to get him through college. He has degrees from the Monterrey Institute of Technology and the University of Pennsylvania, both in economics.

Before being named candidate, Colosio directed the government's huge Solidarity Program, which built roads and brought electricity and running water to remote communities.

Critics called the program, with its $3 billion-plus expenditures, a patronage machine designed to perpetuate the PRI.

He was married to Diana Laura Riojas Reyes, an economist. They had two children, newly born Mariana and Luis Donaldo, 7.

Colosio was so tense that in the two years he was secretary of social development, friends say he had a therapist come in, sometimes several times a day, to give him relaxing massages.

The Colosio family is of Italian origin, coming to Mexico 200 years ago. The family has lived in the western, cattle-raising state of Sonora for at least a century.

Colosio, the oldest of six children - four girls and two boys - spent his childhood helping his father herd cattle and do other chores. While in grade school, he took a job distributing newspapers so he could buy a bicycle.

Like President Clinton, who was inspired by shaking John F. Kennedy's hand, Colosio decided early to go into politics when he went to Mexico City and shook the hand of President Adolfo Lopez Mateo.

The trip was a high school prize for having the best grades throughout.

Colosio astonished then-Education Minister Jaime Torres Bodet, a leading poet, by praising and then reciting without hestitation Bodet's long epic ''El Reloj'' (The Clock).