N orman Rockwell, the artist-illustrator who gained a nationwide following for his many Saturday Evening Post covers, is back in the news 35 years after his death in 1978. Sotheby's in New York is planning to auction seven Rockwell paintings on 4 December, and the paintings are expected to sell for record prices.

According to Sotheby's, one of the paintings, Saying Grace – showing an older woman and a boy saying grace in a crowded restaurant while two men seated at their table gawk at them – could bring in as much as $20m. The previous high for a Rockwell painting, $15.4m, was set in 2006 for Breaking Home Ties, a picture of a teenage boy going away to college.

At a time when the art world is still feeling the effects of the recession, it's understandable why there should be such interest in the prices buyers are willing to pay for Rockwell's depiction of "homey, small-town America" – as one report on the Sotheby's auction put it. The problem with this focus on the dollar value of Rockwell's most nostalgic paintings, though, is that it undermines his greater importance and influence.

We forget that, in the dark days of the second world war, Rockwell played a critical role in helping Americans on the home front understand what was at stake in the fighting going on in Europe and the Pacific. The British had Lawrence Olivier reminding them of their heroic past with his Henry V of 1944. Americans had Rockwell reminding them of their basic decency.

Nowhere is Rockwell's achievement clearer than in the second world war era paintings marking their 70th anniversary this year – Rosie the Riveter and the Four Freedoms, the series Rockwell did illustrating the "four freedoms" that President Franklin Roosevelt declared were the bedrock of a democratic society.

In Rockwell's hands, the Four Freedoms cease being merely abstract principles. They become a familiar way of life, ideals worth defending: Freedom from Want shows a family sharing a Thanksgiving meal; Freedom from Fear portrays parents tucking their children into bed; Freedom to Worship consists of close-ups of people of different faiths praying; and Freedom of Speech centers on a town meeting in which a man – who looks much like a beardless Abraham Lincoln – has his say while his neighbors respectfully listen. These paintings, done in a muted palette, reflect Rockwell at his most serious.

Americans immediately took to the paintings, and in April 1943, the Four Freedoms began a nationwide tour in which over 1.2m people viewed them and also bought $132m-worth of war bonds. At a time when families planting victory gardens in their backyards accounted for 40% of the vegetables grown in the United States, the Four Freedoms confirmed how the decisions Americans made in their everyday lives mattered. When the paintings came to New York, they were not confined to a museum; they were put on display at Radio City Music Hall.

With his painting of Rosie the Riveter – the 29 May 1943 cover of the Saturday Evening Post – Rockwell continued to emphasize the contribution everyone had to make as long as the war lasted. The replacement of men by women in the workforce began well before Rockwell ever produced his rendering of Rosie the Riveter. There was even a 1942 song that declared:

She's making History Working for victory Rosie the Riveter.

Rockwell's genius was to both make Rosie appealing and focus on women who were doing hard manual labor making the weapons of war, rather than just clerical work. In a period in which blonde Betty Grable in a bathing suit was the pinup ideal, Rosie was Betty's physical opposite. In Rockwell's painting, Rosie is wearing work clothes, her arms are muscular, and she is smudged with grease. Yet, Rockwell's Rosie is unburdened by her situation.

We see her taking a lunch-time break, and she appears happy with the choice she has made to help the war effort. Her half-shut eyes give her a dreamy look. She holds a sandwich in her left hand, and she deftly balances a rivet gun on her lap. Any woman, the painting says, could follow in Rosie's footsteps and take pride in what she was doing.

We can only guess at what Rockwell would say today about being pigeonholed as folksy, but we do know how anxious he was to do his part for the war effort. In an interview that he did for a 1945 profile in the New Yorker, he complained of initially being turned down by government officials when he went to Washington and tried to interest them in the charcoal sketches that became the basis for the Four Freedoms. It was the Saturday Evening Post that saved the day by commissioning the Four Freedoms paintings – and allowing Rockwell seven months to complete them, after he had promised to get them done in two.

In the 1960s, when the civil rights movement was experiencing some of its worst violence, Rockwell refused to sit on the sidelines and watch. He directly engaged the conflict with his art, and completed one of his most poignant final works – the Problem We All Live With, a painting for Look magazine. The work depicts a young African-American girl, six-year-old Ruby Bridges, being escorted to a formerly all-white school in New Orleans by a cadre of US marshalls protecting her from an angry and violent mob.

Rockwell never doubted that his depiction of the Four Freedoms would touch people, and in his work during his later years, he made a point of using his art to extend the values embodied by that piece. From the fight against fascism abroad to the fight for freedom at home, Norman Rockwell highlighted the brightness of the American spirit in the darkest of times.