Our year of celebrating Supreme Court Justice Ruth Bader Ginsburg reaches a crescendo with “On the Basis of Sex,” a thrilling if somewhat conventional celebration of her early life and her path to the case and cause that made her reputation.

Mimi Leder’s brisk, upbeat film uses Ginsburg’s biography as a civics lesson, showing the courts as open to powerful arguments and reason (at least in the 1970s), as a reflection of a changing society — not so much leading social change as accepting it — and as Americans’ last line of defense against unfairness, injustice and despotism.

Timely? You bet. Seeing this you understand the fatal attraction of a political party wanting to steal Supreme Court seats and stack the courts to cover its tracks. Congresses and presidents are fleeting, the courts impact generations.

Felicity Jones looks little like the justice, but grows into the part as we watch this razor sharp Brooklynite rise to the occasion of her life’s work. By the time she’s making oral arguments, Jones (“Rogue One”) IS Ginsburg.

The hook that “On the Basis of Sex” hangs on, the thing that got Ginsburg’s cooperation with the film I dare say, is its depiction of a marriage of equals. She got into Harvard Law (and finished at Columbia) and onto the Supreme Court. But husband Marty Ginsburg (Armie Hammer) got into Harvard first, and a mid-law-school medical crisis put Ruth in a position of attending lectures for his courses as well as hers — doubling her education even as she was facing a crushing workload.

When Ruth, who became a law professor because no New York law firm would hire her, was hunting for the test case that might unravel sex (gender) discrimination in the nation’s courts, clever tax attorney Marty was the one who found the ingenious, back-door suit that, with his help, allowed her to change America forever.

“On the Basis of Sex” begins in the sexually-segregated 1950s, with Ginsburg’s first days at Harvard Law. Professors (Stephen Root) refuse to call on her in class. The welcome from the dean (Sam Waterston, perfect) to the nine women of her class repeatedly refers to them “taking a spot from a man.”

Ruth doesn’t need to state the obvious, but she does — “He’s not going to take me seriously.”

She’s a wife and new mother, and gives us a hint of the spitfire she will be as she outworks, out-argues and outflanks the rank sexism of hidebound Harvard.

Casting the tall, patrician Hammer hammers home Ruth’s diminutive stature and “non-traditional” marriage (by 1950s standards) they represented. Marty’s not just a sight gag or sidekick. He’s the better cook, the one who gets hired at a top firm and the one who requires nursing when he has a cancer scare. What doesn’t break Ruth makes her stronger.

The discrimination doesn’t end with her degree. One potential employer lays out the hopelessness of her pursuit of a job in New York law — “a woman, a mother and a Jew.” So she went into academia where by 1970, her protesting/rallying/agitating young students inspire her to get involved — legally — in the spirit of the changing times.

A nice parallel development — the outspoken legal academic discovers the rebel her teen daughter (Cailee Spaeny) has grown into, adding urgency to Ginsburg’s cause.

Veteran character actor Chris Mulkey (“Whiplash”) plays the skeptical litigant whose tax deduction lawsuit gets Marty’s attention, and then Ruth’s. His transformation from haggard taxpayer wronged by a sexist system into social justice warrior beautifully illustrates Ginsburg’s powers of persuasion.

Oscar winner Kathy Bates is the aged, jaded crusading attorney who has fought and lost gender discrimination suits for decades, and Justin Theroux transforms himself into Ginsburg’s pal since childhood who now runs the ACLU’s legal department, a key ally as they begin their assault on “tradition” that has kept women as second class citizens — but a hardcase who is sexist in ways he cannot see.

The villains may make what sound like reasonable arguments against “change” and “preserving the family” — arguments we hear echoed to this day. But Waterston and Root especially show how they deserve our hisses. Leder lets the courtroom arguments have the drama that comes naturally to the setting. Having mother and daughter argue at home over the ethics of Atticus Finch in “To Kill a Mockingbird” underlines the conventions this script is leaning on, to good effect.

“Women have been losing the same argument for 100 years!”

If there’s a knock “On the Basis of Sex” (opening Dec. 25), it’s that the script plays it awfully safe most of the time, relying on Ginsburg’s own words to make her arguments for her and to us. Hiring a totally untested screenwriter doesn’t so much harm the picture as limit its ambitions to the conventional “feel good” moments.

But Jones, luminous in support in such dramas as “The Theory of Everything,” carries this picture, delivers thrilling arguments thrillingly and puts a warm, human face on a legal figure who has become liberalism’s Obi Wan Kenobi, “our only hope.”

“On the Basis of Sex” shows Ruth Bader Ginsburg not as an icon, but as somebody who fought for herself and the rest of us and earned that distinction, one brilliant argument at a time.

MPAA Rating: PG-13 for some language and suggestive content

Cast: Felicity Jones, Armie Hammer, Sam Waterston, Stephen Root, n Mulkey and Kathy Bates

Credits: Directed by Mimi Leder, script by Daniel Stiepleman.  A Focus Features release.

Running time: 2:00