(CNN) A half century after Curt Flood's defiance paved the way for free agency in professional sports, his surviving children are lobbying for their once-estranged father's induction into the National Baseball Hall of Fame in Cooperstown.

"My dad lost his health and his way," said Curt Flood Jr., who's 59. "It was a long road back and, just as he was coming full circle and being of service to his community and young ball players, he became ill with throat cancer and died at the same age I am today."

On Christmas Eve 1969, the former All-Star center fielder for the St. Louis Cardinals wrote a brief but momentous letter to then Commissioner of Baseball Bowie Kuhn challenging the league's stranglehold over its players.

"After 12 years in the Major Leagues, I do not feel that I am a piece of property to be bought and sold irrespective of my wishes," the letter read in part, with Flood not accepting his team's right to trade him to the Philadelphia Phillies.

His rejection of the reserve clause was the advent of free agency, which gave professional baseball players more control over their careers and allowed them to reap the massive contracts many players receive today.

It also effectively ended Flood's career, leaving him a broken man. He drifted apart from his five children and battled alcoholism and money problems. He was shunned by MLB, sports writers and even former teammates.

"He had been ostracized by the game he loved," said daughter Shelly Flood, 58, a drug and alcohol youth counselor in Los Angeles.

'No doubt Curt Flood should be a Hall of Famer': former tennis star Martina Navratilova

Now, nearly 23 years after his death in January 1997, Flood's children and others are heralding what they call his heroic sacrifice in a push for his enshrinement in Cooperstown, New York.

"If Cooperstown is the place where the history is kept, then in the absence of Curt Flood ... there's a continuity issue," his son said.

Flood's surviving children are behind a lobbying campaign to spread the word about his legacy to professional athletes and their unions, sports agents, entertainment figures, journalists and younger fans not familiar with his story. Flood is already in the Missouri Sports Hall of Fame and the Cardinals Hall of Fame.

He can be inducted to the Hall of Fame only through its Golden Days era committee, which meets every five years. The panel of HOF members, executives and veteran media members selects players or figures from 1950 to 1969 who are no longer eligible for regular induction. It meets in December 2020 to vote on inductees for the following year.

On Christmas Day, former tennis star Martina Navratilova tweeted her support.

"No doubt Curt Flood should be a Hall of Famer- I assumed he was there already. Do the right thing #MLB," she wrote.

50 years ago yesterday, Curt Flood declared himself to be an #MLB free agent. His letter would lead to a Supreme Court case and prove to be a major milestone in the development of free agency in pro sports. Curt belongs in the Hall of Fame. https://t.co/zpkw4ohPIh — Billie Jean King (@BillieJeanKing) December 25, 2019

That same day, Billie Jean King tweeted: "50 years ago yesterday, Curt Flood declared himself to be an #MLB free agent. His letter would lead to a Supreme Court case and prove to be a major milestone in the development of free agency in pro sports. Curt belongs in the Hall of Fame."

Supporters point to the Hall of Fame election earlier this month of Marvin Miller , the executive director of the Major League Baseball Players' Association, as well as the union's attorney when the Flood stood up to MLB.

Miller backed Flood's historic challenge of the reserve clause, which essentially tied players to their team forever. After living through Jim Crow discrimination while playing in the minors in the 1950s, Flood fought being traded to Philadelphia without his consent. Miller, a longtime thorn in MLB's side, died in 2012.

'A well-paid slave is nonetheless a slave,' Flood told Howard Cosell

Sportswriters at the time scoffed at Flood's assertion the reserve clause enslaved him.

Sportscaster Howard Cosell once asked Flood how someone earning $90,000 a year -- one of the top salaries in the game then -- could feel like a slave. "A well-paid slave is nonetheless a slave," he responded.

Flood's legal case is considered a turning point in the union's move to eliminate the reserve clause.

In 1970, management agreed to an independent arbitrator to settle future disputes with the union as Flood's suit moved through the courts. Despite Flood's legal defeat in a case that reached the US Supreme Court, Miller forged ahead.

The reserve clause was finally thrown out by the arbitrator on December 23, 1975.

"Today free agency is on the lips of everybody," Shelly Flood said. "It's a part of common conversation."

When pitching ace Gerrit Cole put on a New York Yankees jersey this month for the official announcement of his $324 million contract, he thanked Miller and Flood and "all the players that have sacrificed for us to get in this position."

"A lot of people are saying his name," Shelly Flood said. "When you're a question on Jeopardy, everybody knows."

Still, Flood's son and daughter recall a flawed human, not just a man known for courageous resistance, two World Series titles or three All-Star Game appearances.

After his first marriage, with two adopted children, two of his own and his wife pregnant, Flood leased a ranch-style house in an all-white California suburb outside of Oakland. But the property owner barricaded himself in the house with a shotgun after learning the Flood family was black, according to Flood's son.

Flood sued and his family took possession of the home with an escort of US marshals.

"It was all on TV," Curt Flood Jr. said of the incident after the 1964 World Series, calling it the beginning of his father "resolving horrible situations" with court intervention.

'There was a sense we didn't need him anymore,' Curt Flood Jr. says of his father

Flood's marriage to his first wife, Beverly Collins, fell apart and they divorced in 1966. The family moved to Los Angeles about a year later. Collins later remarried an entertainment lawyer. Curt Flood Jr. and his brother visited their father at spring training in 1968.

"He was always gone," Shelly Flood said in a phone interview. "He was a baseball player. His life was in St. Louis. He had businesses there. He was a part of the St. Louis fabric whereas we were in California. When he was working, he was in St. Louis. When he was off season, he was still maintaining what he had there even though we were here."

In 1969, when she was about eight, her father walked away from the game forever.

"I remember the hush-hush talk about what was going on," she said. "My whole program changed in terms of no longer being able to walk home from school. I had to be picked up because there had been threats against my father and his family."

Flood sat out the 1970 season. He was 32 then. He played his final 13 games in 1972 for the Washington Senators before retiring. That same year he said goodbye to his children and moved to Europe, living in Spain and Denmark until about 1978, according to his son and daughter.

Curt Flood Jr. said his father stopped by their home. Their mother had remarried and they lived in big house with a swimming pool in an affluent neighborhood.

"I think that kind of bruised him," he said. "There was a sense that we didn't need him anymore. That was a breaking point with his spirit. He wasn't the greatest dad anyway in terms of being there and being present."

Flood took the children to a toy store and asked them to get whatever they wanted. He did the same when he returned from Europe a few years later.

"He was a perpetual giver of gifts," Curt Flood Jr. said.

"Only he can say why he did that," Shelly Flood said of the store visits. "For a kid, it was for every disappointment. Every Christmas missed. Every birthday missed. This is how he had to do it."

She added, "When he came back from Europe, he was fragile and empty and probably scared and starting over."

Curt Flood Jr. said his father took him bowling about a year after returning to America and unexpectedly gave his son his World Series ring.

"He said, 'I'm sorry,' '' Curt Flood Jr. recalled. "There was no pomp and circumstance. It wasn't ceremonial. He just handed it to me across the table and we continued to bowl."

Curt Flood married actress Judy Pace in 1986. They were married when he died in 1997. He was still trying to get closer to his children.

"We had begun to try to mend the fence and our relationships individually," his son said. "We each tried to make sense of the circumstances surrounding what happened. We tried to forgive and hopefully learn from it."