The football season has ignited a loud controversy over the country’s most popular sport and the harm it can do to those who play it. Even veteran sportswriters are talking about keeping their kids from the game.

I wonder if it isn’t a good time to remind ourselves why this admittedly rough sport, especially for those who hope to play at the college level, is so important to young men.

For the last several years I have been working on a book about Robert F. Kennedy, trying, like all biographers, to find what drove that raging spirit in the man from the time he ramrodded his brother Jack’s campaigns to that last, desperate presidential run of his own.

What I’ve discovered is the life-changing effect of his two years playing on the Harvard varsity football team. The difference it made in Bobby’s personality was night and day.

In the spring of ’46, before his first season of college football, Bobby was a loner, a quiet, sensitive brooder, totally awkward with people. When he showed up to volunteer in Jack’s campaign for Congress, his older brother couldn’t wait to get him out of sight.

“It’s damned nice of Bobby wanting to help,’’ Jack told a friend, “but I can’t see that sober, silent face breathing new life into the ranks.’’ To keep the brother he called “Black Robert’’ from killing the campaign’s morale, he sent him off to work in some hopeless East Cambridge wards where he couldn’t do much harm.

Compare that with the gung-ho 26-year-old who grabbed hold of Jack’s 1952 senate campaign. Asked to take command, he made that effort a fighting force that was able to topple the estimable Senator Henry Cabot Lodge.

What happened in between?

As his friend and biographer Arthur Schlesinger Jr. put it, Bobby’s “real field of concentration at Harvard was football.’’

That 1946 Crimson team that Bobby Kennedy joined that September was serious business. It went 7 and 2 that season, at one point reaching 18th in the national rankings. The team was bolstered by a good number of returning World War II vets. They were a fairly rough crowd, many of them having served in hard combat.

Kenneth O’Donnell, who’d served as a bombardier in many missions over Europe, was an example. O’Donnell, who was from Worcester, where his father had coached at Holy Cross, watched Kennedy fight his way onto that team.

At 5 feet 8 inches and 165 pounds, Bobby was a not a likely candidate.

“I can’t think of anyone who had less right to make varsity than Bobby,’’ O’Donnell said. “If you were blocking him, you’d knock him down, but he’d be up again going after the play. He never let up. He just made himself better.’’

Even as he competed for his slot on the team, he wanted to be accepted by its members. He quit the prestigious Spee Club and spent most of his waking hours hanging out at the Varsity Club.

Late that November, the season over, he took a bunch of his surly pals home to Hyannis Port for the weekend. His father, who’d once dismissed his third son as a “runt,’’ now saw himself surrounded by guys he was not used to his sons befriending, all helping themselves to his liquor and cigars.

The other person impressed by the sight was his brother, the newly elected US congressman.

“Jack seemed quite astonished that this rough, tough crowd were not only Bobby’s friends but that Bobby fit in with us so well,’’ O’Donnell recalled.

His once-drippy younger brother was not just a friend of these jocks. He was one of them.

Bobby had built a place for himself. That next season, his senior year, Bobby scored a touchdown in the 52-0 swamping of Western Maryland. Despite a broken leg that kept him off the field for most of the season, the coach rewarded his grit by letting him play in the Yale game, which won him his letter.

“He’d had a broken leg!’’ his future wife Ethel would say decades later. “He had a cast on his leg. He really wanted to get his letter.’’

But besides that “H,’’ what was it that was driving him? Was it that he’d missed his chance to prove himself in the war? Was it that dread question: “Where were you in the fighting?’’ Was it the approval of a father who’d harbored his attention on his older brothers, seeing Joe Jr. and then Jack as future presidents?

Whatever it was, the effects were real. For better or worse, the gridiron world he’d chosen had changed him. The quiet, sensitive little brother was now the tough guy. It was during his brother’s stirring ’52 campaign when Bobby won the reputation as “ruthless.’’

“Every politician in Massachusetts was mad at Bobby after 1952,’’ Jack said in knowing appreciation, “but we had the best organization in history.’’

Much has been said about how his brother’s assassination a decade later softened Bobby, made him less the political warrior, and more the tribune of society’s overlooked.

But it wasn’t the first transformation in him. Making the Harvard varsity, and winning that letter, is what toughened him up in the first place, and prepared him to take a chaotic campaign and build it into a fighting force.

It shouldn’t surprise us that the changes wrought by his gridiron days were not always for the better.

The personalities we cheer on the field, after all, aren’t so much fun to live with the rest of the week. A player’s celebrity is easier to take from three rows back in the stands than it is in the dorm or dining hall. Oafishness and entitlement can often be a nasty brew.

But for the right young man, the experience of making the college varsity can be a true rite of passage. College football is not only for those who want to go pro. It’s also for those who simply want to prove themselves.

Chris Matthews is the author of “Bobby Kennedy: A Raging Spirit,’’ to be published Oct. 31. He is the host of MSNBC’s “Hardball.’’