A fully budded marijuana plant ready for trimming is seen at the Botanacare marijuana store ahead of their grand opening on New Year's day in Northglenn, Colorado Thomson Reuters By Scott Malone

BOSTON (Reuters) - Two groups that had backed competing ballot initiatives to make recreational use of marijuana legal in the state of Maine agreed on Monday to join forces in support of just one measure they want to put before voters in 2016.

The Campaign to Regulate Marijuana Like Alcohol said that it would put its organization behind a ballot initiative submitted by Legalize Maine.

Legalize Maine had already collected about 40,000 signatures of voters who support the initiative and would need a total of 61,000 by January to place the measure on a statewide ballot.

"Either one of our measures would have passed but all of us had our doubts about what would happen if there were two measures on the ballot," said Lynne Williams, general counsel for Legalize Maine, whose members played a key role in legalizing medical marijuana in the state. "It's probably always better to work together than to work on parallel tracks."

David Boyer of the Campaign to Regulate Marijuana Like Alcohol, added, "Our initiatives were largely similar overall."

Maine is one of a half-dozen U.S. states where marijuana legalization advocates have submitted initiatives for the 2016 ballot. Voters in Ohio will weigh in on legalization next month.

Attitudes about marijuana in the United States have changed markedly over the past few years, with people in Colorado and Oregon voting to legalize its recreational use in 2012. Since then, voters in Washington, Alaska and the District of Columbia have followed suit.

A Gallup poll released last week found that 58 percent of U.S. residents believe marijuana should be legal, with the highest support among respondents aged 18 to 34.

The Campaign to Regulate Marijuana Like Alcohol is backed by the Marijuana Policy Project, a national group that is also supporting a measure the Campaign is pushing in Massachusetts.

There the Campaign faces competition from three initiatives submitted by Bay State Repeal. The Campaign's Massachusetts initiative, which is similar to its Maine proposal, calls for special taxes on marijuana, a provision opposed by Bay State Repeal.

Members of those two groups on Monday indicated they were unlikely to join forces any time soon.

"It would be very nice if they agreed that ours is a better law," said Steven Epstein, of Bay State Repeal.

Will Luzier, of the Campaign in Massachusetts, said: "Our proposal is divergent from the proposal of the other group ... It's doubtful that we would be able to combine forces here in Massachusetts."

(Reporting by Scott Malone in Boston)