The measure would allow patients whose doctors say they have six months or less to live to obtain lethal doses of medication.
Thirty-four states prohibit assisted suicide outright, while Massachusetts and six others ban it through common law. Oregon and Washington allow it.
A September Suffolk University/7 NEWS poll found 64 percent of likely Massachusetts voters support the initiative.
Religious, medical and disability rights groups are fighting the measure, saying it’s open to manipulation and relies on diagnoses that may be wrong.
Supporters, primarily patients’ rights groups, say the bill has effective safeguards, including prohibiting doctors from prescribing the drugs to depressed patients.