IN THE LATE '60s, he was a cocky bantam rooster among the civil rights activists, this slightly-built Roman Catholic priest. It was a time for militant causes, and the then Rev. James E. Groppi was not a man to turn his back on a cause. Rather he embraced it with fervor and dedication.

He was jailed more than a dozen times. He led 200 nights of marathon marches for open housing through taunts and stones thrown in Milwaukee's Polish-American South Side. He came to Washington with a bodyguard of "black avengers" from the Milwaukee NAACP Youth Council, which he advised.

The marching priest was page-one news whether photographed in jail or testifying before the president's riot commission.

Today the former priest-activist is Jim Groppi, a Milwaukee bus driver on strike and depending on his wife to bring in the income after more than 25 days off the payroll.

"Some of the fellows have been talking about my running for president of the union," Groppi said last week. "But I don't know. We're beginning to hurt."

There is a flash of old fire. More than a decade after the heady excitement of the civil rights demonstrations, Groppi still can work up a good case of righteous indignation. He's still searching in his personal life, he says, but that doesn't mean that he is lost or forlorn.

A little over two years ago, Groppi was dropped from the priesthood when he was married at the age of 45. Peg, his wife, was a parishioner at St. Boniface Catholic Church when Groppi was the priest there. They were married in Las Vegas, Nev.

"Peg's been in jail too during the civil rights demonstrations," Groppi points out with the pride of a wife's accomplishment. "It was an agonizing time for me as I considered getting married. In the church, you had a home. As a priest, a lot of people were depending on you. You could feel that you were accomplishing something. There was structure and dedication."

There still is dedication if not the buttress of a structured discipline for Groppi. He and his wife now are volunteer parish workers for St. Andrew's Epiacopal Church in Milwaukee and help out on neighborhood canvases.

"It's good to knock on doors again and most people. I say 'I'm Father Groppi' and talk to them. I don't care what people call me, but I still use 'father," from habit," says the former priest.

Always a maverick within the church, Groppi was disciplined by his superiors during his militant days as a civil rights demonstrator. He finally was transfered from his inner-city black parish to the suburbs. Later he set up a storefront church and celebrated mass. Then he was given leave as a priest to attend law school and earned tuition money driving a cab in Milwaukee.

"I was helping people just as if I was in the church. As a cab-driver, I was out there with real people in the world," Groppi recalls. "Bus driving is also being where life is."

After his marriage, Groppi applied for 25 jobs either in social work or Episcopal parish work. His application to become an Episcopal priest was rejected because of an agreement between the Roman Catholic and Episcopal dioceses that neither accept the transfer of a priest from the other.

"Sure I was a little disappointed," the former priest conceded. "I'm still looking for a church parish. It's an investment of 25 years of my life. I was ordained at 29 after nine years in the seminary. I don't see where it makes any difference whether a priest is single or married. The most effective clergyman of our time was Martin Luther King, a married man."

Groppi goes to daily mass at St. Andrew's Episcopal church with 15 or 20 former Catholic priests who now are married. One of them left the priesthood after marriage at 55 with only $5,000 savings and no job.

"It can be a very traumatic experience," Groppi emphasized. "They even wiped out his retirement savings in the priest's fund."

Groppi sometimes gives talks on optional celibacy for the clergy. He feels changes will come, if slowly, in the Catholic Church's attitude toward married priests and the status of women. But he's not sure he would return to the fold.

Groppi's wife, Peg, who has a doctorate, is teaching in the higher education department at a university.

"Peg's a provider as most wives have to be today," Groppi says. "She sometimes refers to herself as a 'contaminant' in the eyes of the church when she married me. One day I'm Father Groppi and the next day, after marriage, I'm 'Jim.'"

Groppi still finds the world full of causes and issues of concern. "I'm really fightened about the African situation. It sounds like Vietnam over again." But he also worries about the month-long bus strike and the rent payment.

And then there is that book that he was going to write "but never tied into."