Even the most ardent supporter of Sen. Bernie Sanders has to wonder: How much of his aggressively progressive agenda could he expect to achieve in Washington? How could a self-described democratic socialist carry the day amid a Republican Congress that stymied President Obama’s initiatives at almost every turn?

Sanders invokes the lessons of history in making the case that his presidency could propel a sharp leftward shift in a country that has been deeply polarized for a generation.

“What I believe is the way we make change today is the way that change has always taken place in this country, whether it is the rise of the trade unions and workers’ rights, whether it’s the civil rights movement, whether it’s the gay movement or the women’s movement,” Sanders said in a meeting with our editorial board Tuesday.

“It never takes place from the top down. It’s always from the bottom on up.”

Sanders’ premise is that his bold leadership would galvanize Americans to become politically engaged “in a way that doesn’t exist now.” Voters would demand that Congress raise the minimum wage to $15 an hour, offer free tuition at public colleges and deal with “the obscenity of the grotesque level of income and wealth inequality” in the nation.

Lawmakers would resist his programs at their peril, Sanders suggested.

“Republicans and Democrats are not dummies,” he said. “They know where the votes are.”

Many American presidents have come into office with dreams of an unstoppable mandate. Few have succeeded.

“What Bernie Sanders is talking about is a New Deal revolution like FDR,” said historian Douglas Brinkley, a Rice University professor and preeminent scholar on the American presidency.

Franklin D. Roosevelt came into office after defeating President Herbert Hoover by nearly 18 percentage points and 413 electoral votes. Those numbers would be unthinkable in the contours of contemporary politics, with American elections routinely decided in a handful of battleground states.

And there was that other factor that created the climate for the “wild freedom that FDR had for not just 100 days, but for almost a year,” Brinkley reminded.

“The nation was completely and utterly crippled by the Great Depression,” Brinkley said in a phone interview. “We’re not that right now.”

While the rich-poor divide and the sense of lost opportunity that is attracting young people to Sanders’ message is very real —as is the concern it presents for the nation’s long-term stability — it does not compare with the desperation of 25 percent unemployment in 1932.

“Great groundswells that propel presidents are based on national events such as as war or depression/recession,” said Stephen Hess, a senior fellow at the Brookings Institution whose specialties include the presidency. “I think the country is in a middling moment, but not feeling the type of threats that will propose a groundswell for a new president.”

Two other presidents who reshaped government in significant ways — Democrat Lyndon B. Johnson and Republican Ronald Reagan — also enjoyed the tailwinds of landslide electoral margins. Johnson had the additional push of public sentiment to fulfill the vision of his slain predecessor, President John F. Kennedy; Reagan reached the White House when Americans were reeling from a “misery index” (inflation plus unemployment rates) of nearly 20 percent. Today it is less than 6 percent.

Sanders is quick to complain that “the establishment” is against him when confronted last week with nonpartisan studies that suggest his programs, from government-paid tuition to universal health care, would nearly double the nation’s $18 trillion debt in a decade.

The history of the American presidency suggests he would be extremely hard-pressed to enact his progressive revolution.

John Diaz is The Chronicle’s editorial page editor. Email: jdiaz@sfchronicle.com Twitter: @JohnDiazChron

It takes a groundswell — and then some

These three presidents were able to leverage landslide victories, and a climate distinctly receptive to radical change, to get their ambitious agendas through Congress.

Franklin D. Roosevelt, 1932

Landslide: 57.4% of popular vote; 472 of 531 electoral votes against incumbent Herbert Hoover

Agenda: In his first 100 days, FDR successfully shepherded 15 major bills through Congress, dramatically expanding the scope and reach of the U.S. government. His initiatives ranged from imposing the first federal regulations on the stock market to initiating the American welfare state.

Contributing factor: The Great Depression. The stock market had dropped 85 percent from its 1920 high, and 1 out of 4 American workers were unemployed.

Lyndon B. Johnson, 1964

Landslide: 61.1% of popular vote; 486 of 538 electoral votes against Sen. Barry Goldwater

Agenda: LBJ successfully built on FDR’s New Deal with an ambitious further expansion of federal programs, many aimed at eliminating poverty and racial injustice, in what he called the Great Society. He also helped prod passage of the long-stalled Civil Rights Act.

Contributing factor: The 1963 assassination of John F. Kennedy created public determination for his successor to carry out his vision of a New Frontier.

Ronald Reagan, 1980

Landslide: 50.5% of popular vote; 489 of 538 electoral votes against incumbent Jimmy Carter

Agenda: He pushed through tax and spending cuts, along with a serious curtailment of government regulation in everything from the environment to consumer protection, that changed the course of a government that had been growing in size and power since FDR.

Contributing factor: Reagan’s electoral landslide gave Republicans control of the Senate for the first time in a quarter century. His wounding in an assassination attempt two months after taking office helped build sympathy and an aura of invincibility for the new president who was known as the Great Communicator.