People gather during a rally against efforts by Republicans in Congress to repeal the Affordable Care Act and in support of a single-payer health-care system in New York City in July 2017. (Justin Lane/EPA)

Bill de Blasio, a Democrat, is mayor of New York City.

When a party loses a national election to someone such as President Trump, it’s time to rethink everything. But with just a year from the Iowa caucuses, I’m worried some of our leaders in the Democratic Party are learning the wrong lessons.

Some are saying that bold ideas such as Medicare-for-all, a job guarantee and free college sound great — but aren’t realistic.

“When the stakes are another four years of Trump degrading our country, do we really want to use the 2020 campaign as a first-time experiment on idealistic but unrealistic policies?” wrote former Virginia governor Terry McAuliffe in The Post this month.

McAuliffe is right that we should not overpromise. But he’s absolutely wrong that a bold vision of fundamental change is more than we can deliver. We progressives are all too familiar with his argument. It’s a lie we’ve been told by the members of our own party and the mainstream media for far too long.

When I first ran for mayor in 2013, the New York Times praised my “ambitious plans” but urged voters to support a different candidate because my ideas looked “like legislative long shots.”

The common belief is that you are either a dreamer or a realist. But idealism and pragmatism aren’t as far apart as one might think.

I won that race in 2013 because of the grass-roots power that pundits and pontificators cannot understand. We put forward a clear, populist message: New York City was a tale of two cities, with unacceptable levels of inequality that had to change. The voters agreed.

We took from the great example of Franklin Delano Roosevelt — that you must make change people can feel and must do so quickly. We implemented prekindergarten for all, we created an ambitious mental-health program called ThriveNYC, and we proposed the biggest affordable-housing plan in the city’s history. Meanwhile, crime fell to record lows when we ended the era of stop-and-frisk and created neighborhood policing.

Last week, I announced a similar policy in that same mold: We will deliver guaranteed health care for all New Yorkers. If one of our residents is insurable, we’re going to enroll her in New York City’s public option. If she can’t afford insurance or she’s ineligible, we’ve created a way for her to get guaranteed, quality care in a public hospital or clinic.

We’re doing this because it’s the right thing to do morally and medically, and we’re not alone in expanding health care. In Washington state, Gov. Jay Inslee (D) proposed last week the creation of a public option to be sold on the state insurance exchange. Down the coast, newly sworn-in California Gov. Gavin Newsom (D) is also pushing to get more people the care they need.

I’m sure at this point some people are thinking that all this is easy to do in deep-blue places such as New York City, California and Washington. And they’ll note that the 2020 election will require Democrats to win in places such as Florida, Michigan and Missouri, where convincing voters to support such ideas will be much harder.

The results from the 2018 midterm elections prove them wrong. Trump’s incredibly small margin in Florida in 2016 was clearly helped by the fact that 1.5 million people in the state who were previously convicted of a felony could not vote — even if they’d served their time. But last November, 64 percent of Floridians supported Amendment 4 restoring their voting rights.

In Michigan, 11,000 votes separated Trump and Hillary Clinton when the dust settled. Can you guess the size of the margin when automatic voter registration was on the ballot in 2018? “Yes” beat “no” by 1.4 million.

Trump won Missouri in 2016 by 19 percentage points. Two years later, Missourians voted overwhelmingly to eliminate the state’s “right to work” law. They also approved medical marijuana and sweeping campaign finance reform.

These are exactly the kind of “idealistic” and “radical” changes everyday people demand, and we can deliver — not just in blue states but all over country.

Too many Americans feel like the walls are closing in. They don’t care about returning “normalcy” to our political discourse — whatever the hell that means. Voters want significant proposals for equality and fairness. Anyone who tells you otherwise is dooming our party to repeat the mistakes of the past.

There are many signs that the dawn of a new progressive era is here. Now is the time for our party to redefine the possible. We can only do it by refusing to listen to what centrists tell us is realistic. Look instead at what progressives have already done. It’s time to catch up with the new, progressive future and not allow ourselves to be left behind.

Read more:

Terry McAuliffe: To beat Trump, Democrats must counter his lies with realistic solutions

Paul Waldman: Will the 2020 Democrat be progressive, or electable against Trump? Probably both!

Jim Kessler and Lanae Erickson: Don’t let progressives fool you. Moderate Democrats can win.

Adam Green: The midterms prove it: Progressive ideas are now mainstream

Dana Milbank: Stop the bidding! This Democratic presidential candidate is about to clear the field.