The groundswell for civil rights was growing in 1972 but it took an African American woman, a teacher from Brooklyn, to galvanise the mood for change when she pressed ahead with her campaign for president. This Netflix film is a remarkable true story. It also has a refreshing positivity that defies the gloom that can settle the moment this year's US presidential election lumbers into view.

In this biopic, written and directed by John Ridley, the story emerges of a woman ahead of her time. In the life of the remarkable American politician, congresswoman Shirley Chisholm, there were lots of firsts. Including being the first African American woman to be elected to the US Congress, the first African American woman to campaign for the Democratic Party presidential nomination and the first woman to take part in a presidential debate. An actor with depth and brio is required for a role like this. Shirley Chisholm is played with strength and conviction by Regina King.

With seven months to go until the Democratic convention, Chisholm began her run with a close team including campaign adviser Wesley McDonald "Mac" Holder (the late Lance Reddick of The Wire), Robert Gottlieb (Lucas Hedges), her student coordinator, and Terrence Howard as finance manager Arthur Hardwick Jr. In 1972, the presidential election was open for the first time to 18-year-olds. No doubt for some this contributed to a buoyant anticipation of change, but it isn't what happened, of course, and the incumbent, Richard Nixon, won in a landslide.

Chisholm's election motto, "Unbought and unbossed", hints at everything that both helped and hindered her. There was uncompromising honesty alongside a propensity to want to call the shots. Her husband of 19 years, Conrad (Michael Cherrie), with whom there were no children, was willing to be her shadow, but the rigours of campaigning began to show their marriage was fraying.

It would have been painful for Chisholm that some members of her own family, including formidable sister Muriel, were resentful of her success. In an art-mirrors-life moment, Muriel is played by Regina King's real-life sister, Reina. But Reina, who is also a co-producer on this project, is absolutely on board here.

With her independent frame of mind, the fascinating character of Chisholm, or "Mrs C" as she is known to her associates, has a bracing freshness today. Fifty years on, such frankness probably wouldn't survive in today's sceptical, cynical political arena. It is a tonic to watch King bring Chisholm to life in a script that reflects many of her actual public statements. Chisholm was a terrific public speaker, authoritative and down-to-earth, but she was never too busy to pause for strawberry milkshake.

In what today might look like reckless adherence to principle, she was fearless and outspoken, true as she saw it to the Christian faith that underpinned her life.

The film's running time is long, and it tends to sentimentality. The real Mrs C, who had a way with words and made smart, pithy statements, could have taken to it with her schoolteacher's pencil and made it harder and sharper.

Ultimately, Chisholm's message at the end of her seven-month run for nomination was to remind people of what was possible.

Although she had robust support among the minority communities in New York, it is easy to see that the electorate at large wasn't quite ready for her. Even some prominent feminists weren't. Chisholm claimed she met more opposition to her campaign for being a woman than for being African American, and that is worth reflecting on.