While other Democratic hopefuls support a minimum wage hike and earned sick time, Berwick is the only candidate to make the so-called single payer option a central part of his platform.
The state's landmark 2006 healthcare law dramatically expanded healthcare and, despite its botched healthcare website, Massachusetts still has the higher percentage of insured residents in the country.
But the system, which relies on expanding the existing market-based healthcare model, falls short of the universal coverage envisioned by Berwick and liberals.
"Don is the most progressive candidate in the race with a vision for the commonwealth that includes single-payer healthcare," said Ben Wright, the group's executive director.
Berwick, who headed the CMS for 17 months, has praised the 2006 Massachusetts law and the 2010 federal law that it inspired.
But he said the state and the country can do better by essentially expanding Medicare, which covers the elderly, to include all residents regardless of age.
Berwick—one of five Democrats hoping to succeed Gov. Deval Patrick—will need the support of organizations like Wright's to help close a fundraising gap and ensure he receives the 15% of delegates at next month's convention needed to land a spot on November's primary ballot.
As of May 15, Berwick had $183,778 in his campaign account compared to the two top Democratic candidates for governor—state Treasurer Steve Grossman with $906,545 and Massachusetts Attorney General Martha Coakley with $453,823.
Other Democratic candidates for governor include Juliette Kayyem, who had $189,437 in her account and Joseph Avellone, with $70,947.
The endorsement came as Berwick dropped off nearly 16,000 certified nomination signatures to the state secretary's office. The signatures are also needed to secure a spot on the ballot.
Also Wednesday, the Massachusetts Teachers Association, the state's largest teacher union, declined to endorse a candidate in the Democratic primary.
The group, one of the state's most powerful unions, said it "stands ready to back" the Democratic primary winner over Charlie Baker or Mark Fisher, the GOP hopefuls.