Sen. Kamala Harris checked all the progressive policy boxes in the first week of her presidential campaign.

Does she support Medicare-for-all, single-payer health care? Check.

Debt-free public college tuition? She’s in.

A tax credit for the working class? Yep.

She’s not alone. The Democrats already running for president — and those expected to announce soon — share something in common at this early point of the 2020 race: They can’t sprint fast enough to the left. Most generally support the same progressive platform when it comes to health care, taxes, the environment and education.

Harris amplified her support for government-paid health care for everyone Monday when she was asked during a televised town hall meeting in Iowa whether people who like their insurance plans could keep them under single payer.

“The idea is that everyone gets access to medical care, and you don’t have to go through the process of going though an insurance company ... going through the paperwork,” Harris said. “Let’s eliminate all of that. Let’s move on.”

That kind of support for a solidly progressive cause would have made Harris an outlier in the 2016 Democratic field. Ask Sen. Bernie Sanders, an independent from Vermont who calls himself a democratic socialist.

Sanders was the only candidate then to support government-insured universal health care coverage. His main opponent, eventual nominee Hillary Clinton, didn’t. She also fell $3 short of Sanders’ call for a $15-an-hour federal minimum wage.

Now much of the Democratic field shares the same views as Sanders on a range of subjects, including health care. But subtle differences are emerging.

• Medicare for all: Three years ago, Clinton opposed Sanders’ single-payer health care proposal because she didn’t “want to see us start over again” on overhauling the U.S. health care system “with a contentious debate.”

Since then, Trump and congressional Republicans have weakened the supports for Barack Obama’s signature Affordable Care Act, and the climate for Democrats has changed.

Sanders led the way, introducing single-payer legislation in the Senate in 2017 and identifying several possible ways to pay for it, including a payroll tax on employers and raising taxes on upper-income people. Harris was the first of the other 2020 presidential hopefuls in the Senate to co-sponsor it, quickly followed by Sens. Elizabeth Warren of Massachusetts and Kirsten Gillibrand of New York. Another probable candidate, former Obama administration official Julián Castro, is also on board.

But it’s not unanimous. Ohio Sen. Sherrod Brown, who may run, supports lowering the Medicare minimum age from 65 to 55. He also supports a public option, which could allow younger people to buy into Medicare. Former Vice President Joe Biden, a possible candidate, also has supported lowering the Medicare eligibility age. But neither has backed Sanders’ single-payer plan.

It’s possible the bulk of the likely Democratic field is to the left of the country on the issue. More Americans support lowering the Medicare eligibility age (77 percent) than support Medicare for all (56 percent), according to a survey this month from the nonpartisan Kaiser Family Foundation.

One potential candidate who doesn’t support single payer is Howard Schultz, the former Starbucks CEO who is contemplating an independent run. Schultz told “60 Minutes” that “what the Democrats are proposing is something that is as false as (President Trump’s proposed border) wall — and that is free health care for all, which the country cannot afford.”

• Reduced public college tuition: Sanders proposed eliminating undergraduate tuition and fees at public universities in a 2017 bill. Price tag: $47 billion. He would have paid for it with a tax on financial transactions involving stocks, bonds and derivatives. Harris, Warren and Gillibrand supported his bill, which, in a GOP-run Senate, went nowhere.

Castro found out during his presidential campaign rollout this month that progressives will pounce on candidates who aren’t all in on this issue. Activists criticized Castro for saying that “we’ll work to make the first two years of college ... accessible and affordable” instead of all four years. He later clarified that, saying that “at least the first two years of college or university or apprenticeship program should be tuition free — and preferably four years.”

• Climate change: Sanders supports the Green New Deal, a broad policy proposal backed by more than four dozen House Democrats. It calls for the U.S. to be fully relying on renewable energy within 12 years and to guarantee jobs to people to make the transition happen.

There is no price tag or even an identified funding source. Still, Warren, Gillibrand, Harris and Castro have signed on to the idea, said Stephen O’Hanlon, communications director for the Sunrise Movement, which is organizing the Green New Deal.

• Taxes: Democrats are showing a renewed vigor about raising taxes on the wealthiest Americans and using the money to pay for progressive-backed projects.

Warren introduced a plan last week to levy a 2 percent tax on personal wealth of over $50 million and 3 percent above $1 billion. She would use the revenue to reduce college and health care costs.

In October, Harris proposed a $6,000 annual tax credit for families making less than $100,000 a year. She said she would pay for it by rolling back unspecified parts of the 2017 GOP-sponsored tax law and and by taxing banks that have more than $50 billion in assets.

The underpinning for such plans is a sense that the Democratic rank-and-file is moving to the left. Markos Moulitsas, founder of the progressive DailyKos blog, says supporting single payer is the “price of admission” to the Democratic primaries. Green New Deal backer O’Hanlon says candidates need to back the environmental plan if they want to be “taken seriously by young people.”

But not all Democrats are heading that way. A nonpartisan Pew Research Center survey this month found that 53 percent of registered Democrats want their party to move in a more moderate direction, while 40 percent would rather it moved to the left.

Dennis Goldford, co-author of “The Iowa Precinct Caucuses: The Making of a Media Event,” said, “The Democrats have to be careful about McGoverning themselves” — choosing a candidate who appeals to the party’s base like its 1972 nominee George McGovern, but who is too liberal for the rest of the country.

“Do Democrats want someone who is liberal or extremely liberal?” said Goldford, a professor of public policy at Drake University. “Do they want someone who can make their heart flutter, or do they want someone who will appeal to their head?”

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli