AUGUSTA, Maine — The state’s ethics watchdog on Wednesday voted to investigate potentially illegal polling against a progressive candidate running for the Maine Senate, though getting to the bottom of the responsible group may be a hard task.

A group calling itself Public Opinion Research has potentially been using a campaign tactic called “push polling” against Rep. Chloe Maxmin, D-Nobleboro, who is running against Senate Minority Leader Dana Dow, R-Waldoboro, for a perennially competitive seat representing Lincoln County and neighboring towns.

The poll itself may not be illegal, but spending on the poll has not been reported to the Maine Ethics Commission and the survey does not include language about the group that authorized it. Both Maxmin and Dow have said they do not know who is behind the survey, which apparently attempts to influence voters by saying Maxmin is in league with “radical liberals.”

Those factors make the issue ripe for further investigation, the five members of the ethics commission found on Wednesday after a complaint from the Lincoln County Democratic Committee. But figuring out who is responsible may prove to be the greatest challenge in deciding whether it is illegal or punishable.

Commission staff said tracking down the people behind Public Opinion Research has proven challenging. A telephone number listed at the end of the online survey leads callers to an automated voice system with no option to speak to a person.

The state has a narrow definition of push polling that the survey may not meet. Any group using such methods must report expenditures not coordinated with a candidate if they exceed $250. Group also must register as a political committee if they spend $5,000 on a state election.

Not knowing how much money has been spent on the polling also makes it difficult to know if a violation has occurred, but Jonathan Wayne, the executive director of the commission, said it was likely the surveys exceeded the threshold given past reported prices of polls.

A group with the same name conducted push polling in Tennessee, but the ethics commission in that state has received no complaints. A related Facebook page spent over $100,000 on the presidential campaign, but political committee registrar Michael Dunn said attempts to reach it and another organization listed in disclaimers have also been unsuccessful.