The new poll shows Lt. Gov. Matos in third place, marking a dramatic change from a Matos internal poll in June that showed her in first, leading Regunberg by 13 percentage points.
But voters may find it difficult to identify policy differences in a 12-person Democratic field where the leading candidates agree on many issues, observers say.
Seven of the 12 Democrats running for the First Congressional District seat are Black and/or Latino, presenting a chance for R.I. to elect a person of color to Congress for the first time.
The endorsement by the Vermont independent follows support from the Congressional Progressive Caucus PAC, but some Rhode Island progressives remain sharply critical of Regunberg.