PORTLAND — Supporters of a local referendum to legalize recreational use of marijuana in Maine’s largest city launched their official campaign Thursday, saying Portland can pave the way for a statewide end to prohibition of the drug like Denver did for Colorado in 2005.

Colorado’s largest city legalized possession of usable amounts of pot eight years ago in a step widely seen as a precursor to the statewide legalization in late 2012.

As was the case in Colorado before voters there OK’d recreational marijuana use, the medical use of marijuana is already legal in Maine through government-approved dispensaries and with doctor prescriptions. But with an ordinance change that would allow people in Portland to legally possess 2.5 ounces of the drug, referendum supporters said Thursday, those who need the pain relief offered by cannabis but can’t afford a doctor’s appointment will have access.

For those without medical concerns, the legalization advocates said, the measure would dramatically cut down on the amount of time and money the criminal justice system spends on nonviolent offenders.

The referendum will appear on Portland ballots on Nov. 5 after several local groups gathered more than the 1,500 petition signatures necessary to force a citywide vote on the subject.

Grainne Dunne, justice organizer with the American Civil Liberties Union of Maine, made note of the oft-referenced statistic that Maine spends nearly $9 million annually enforcing marijuana laws.

“This is an opportunity to realign our justice priorities with our values,” Dunne said during the Thursday morning event at City Hall. “That’s money better spent on education, health care or infrastructure.”

Added Knox County medical marijuana grower Will Neil, “It’s obviously insane that we’re wasting tax dollars prosecuting small-scale possession charges.”

Attending the local “Yes on 1” campaign kickoff Thursday along with ACLU representatives were Hillary Lister of the Medical Marijuana Caregivers of Maine, Maine Green Independent Party chairman and campaign outreach director Asher Platts, campaign manager Anthony Zeli, local author Crash Barry and Portland City Councilor David Marshall, among others.

Platts urged supporters in the community to visit the website LegalizeMarijuana.me to donate to the campaign — he said organizers set a $4,500 fundraising goal — sign up for promotional materials to distribute or schedule a “house party” to raise awareness about the referendum.

While ACLU of Maine Executive Director Shenna Bellows noted their modest fundraising aspirations and described the group of legalization backers as “really a Portland-based, Maine-based coalition,” David Boyer of the Washington, D.C.,-based Marijuana Policy Project acknowledged to reporters Thursday his organization is prepared to spend “whatever it takes” to promote passage of the citywide referendum.

But, he added, “if we didn’t drop a cent on this campaign, I think it would pass. I think most people see the value in this.”

Those at Thursday’s news conference made it clear they see Portland as a stepping stone toward a statewide marijuana legalization effort.

Dunne said a “yes” vote at the polls in Portland would “send a powerful message” to state and federal lawmakers.

“What’s happening here in Portland is a very forward-thinking first step of having the government realize the ludicrous nature of our crazy drug policies,” Barry said. “We have a homegrown medicine that adds untold dollars to our state’s economy.”

Efforts to legalize pot possession suffered a setback at the statewide level during the recently completed legislative session, during which a bill that would have legalized recreational use of the drug statewide attracted 35 co-sponsors but did not gain the endorsement of the Legislature’s Criminal Justice and Public Safety Committee. The House and Senate subsequently voted down a follow-up proposal that would have put the legalization issue before voters on statewide ballots in the fall.

Proponents of the legalization measure, including City Councilor Marshall, have said the prohibition of the drug is ineffective and drives use of the substance underground. They say pot use should be allowed, regulated and taxed the same way as alcohol, which was prohibited throughout the United States in the 1920s in what is largely considered a failed initiative.

Opponents of the move, including the Maine Chiefs of Police Association, have argued that legalizing recreational use of marijuana would exacerbate substance abuse problems that feed other crimes, and that overseeing regulation and distribution of pot would be an expensive and time-consuming task for already overburdened state agencies.

The Maine Office of Substance Abuse and Mental Health Services has argued that marijuana today has more than double the mind-altering chemical THC that the pot on the market in 1983 had, making it more potent and addictive, and that teenagers may be more likely to experiment with the drug after hearing legalization advocates downplay the danger.

Efforts to defang enforcement of marijuana laws in Portland in the past have fallen short. In 2011, activists gathered signatures on a petition seeking to make pot possession offenses the lowest enforcement priority for Portland police. But despite getting more than 2,100 signatures on the petition — 600 more than necessary to gain a spot on the local ballot — the city clerk’s office found the document was invalid because only about 1,400 of the names were from verified Portland residents.