Sander M. Levin, a Democrat, represents Michigan’s 9th Congressional District in the House. Henry Waxman, a Democrat, represented California’s 33rd Congressional District in the House from 1975 to 2015.

In all of our years promoting progressive legislative policies in Congress, no vote was more challenging or consequential than the one to pass the Affordable Care Act.

As the former chairs of the two House committees that had primary jurisdiction over health-care reform when the ACA was passed in 2010, we’re proud to have helped realize a long-standing goal of the Democratic Party, moving our nation significantly closer to attaining universal health care.

Thanks to the ACA, nearly 18 million previously uninsured Americans have health coverage. Insurance companies can no longer deny people coverage for preexisting conditions or charge them more just for being women. There are no longer annual or lifetime limits on care, and young people can remain on their parents’ plans through age 26. In short, the ACA stands as a historic triumph for our party and the nation.

President Obama had the grit and courage to make health-care reform a top priority of his administration. And 219 House Democrats, 58 Senate Democrats and two independents — including Sen. Bernie Sanders of Vermont — stood together to forever transform health care in the United States.

But this pivotal bill passed by the smallest possible margin. It wasn’t easy, and we weren’t sure we even had the necessary votes until right before they were cast. Then, in the election that followed, Republicans succeeded in vilifying the new law, and we watched as many colleagues who had taken those courageous votes lost their seats.

Since the ACA became law, we have lost majority control of the House and Senate, while opposition to the law has been constant and implacable. Republicans in Congress, backed by entrenched and ideologically driven interests, have voted to repeal or dismantle the law a mind-boggling 62 times.

Until recently, these votes were merely symbolic. But weeks ago, opponents succeeded in passing a bill to overturn the ACA. Of course, the president wasn’t about to sign a repeal of his signature achievement. But in 2017, a President Trump , or a President Cruz, or any other Republican would do so in a heartbeat. They promise daily to do just that.

That is why we are both speaking out now, forcefully and clearly, as the Democratic presidential primary unfolds.

Our party needs our nominee to emphasize what a breakthrough the ACA was, and how it’s working for millions of Americans and improving our health-care system. We’ll need to convince voters in the general election that defending the ACA is a reason to vote Democratic. The same will be true in key contests in the Senate and House.

That’s why we believe that Sanders’s proposal to throw away the ACA to pursue a single-payer system is counterproductive at best and dangerous at worst.

After years of Democrats holding the line to defend, implement and improve the law, Sanders would surrender the ground we’ve fought so hard to gain to those in favor of repeal.

Ron Pollack, the executive director of Families USA, has said he worries it “moves us away from an effective and practical agenda.” Policy-oriented progressive writers Jonathan Cohn, Matthew Yglesias and Jonathan Chait have each described Sanders’s plan as vague and unrealistic. New York Times columnist Paul Krugman, a Nobel Prize-winning economist, has declared that “for all the talk about being honest and upfront . . . Sanders ended up delivering mostly smoke and mirrors.”

Many worry that relying on funding from income and payroll taxes, as the Sanders plan does, means that many people who receive subsidized coverage today would end up worse off. For example, under the ACA, people on Medicaid and families on health-care exchanges get coverage with strictly limited premiums and caps on cost sharing. Under the Sanders plan, they would bear a new tax burden that, in many cases, could lead to them paying more for coverage than they do now.

We know we still must work toward 100 percent coverage. We need to lower out-of-pocket costs and the price of prescription drugs. We would have liked to see a public insurance option created to increase competition and drive down prices. But the ACA is a major step forward and a foundation for further progress. So let’s not go back to the drawing board.

Ever since Hillary Clinton first testified before Congress in 1993 on the need for the United States to pursue universal health care, she has been fighting this battle. Today, she has put forward real and achievable ideas to keep the ACA moving forward, and that is a major reason we both support her to be our party’s 2016 presidential nominee. We have confidence in her ability to help Democrats complete our great unfinished work.