A ballot measure to legalize use and retail sales of marijuana continues to hold the support of half of Colorado voters in a new Denver Post poll, setting up a campaign finish that may go down to the last vote.

Amendment 64 — which would make legal possession of up to an ounce of marijuana for people over age 21 and allow for marijuana to be sold in special stores — is supported by 50 percent of likely voters surveyed for the poll. It is opposed by 44 percent. The amendment’s 6-point lead is outside the poll’s 3.8 percent margin of error.

In two previous Denver Post polls, Amendment 64’s support has fluctuated above and below 50 percent but has always held at least a 5-point advantage.

That does not mean, though, that passage looks likely.

The Post’s poll found Amendment 64 tied among voters who said they had already cast a ballot. Its biggest lead was among people who said they would be voting on Election Day.

“Passage would be driven largely by the support of younger voters, who sometimes are less reliable, turnout-wise, than are older voters,” the polling firm SurveyUSA, which conducted the survey for The Post, wrote in a memo explaining the results. “Older voters oppose Amendment 64, and if the amendment should go down to defeat, it will be because younger Coloradans talked the talk but did not walk the voting-booth walk.”

Conscious of that scenario, supporters of Amendment 64 plan to spend the final days before the election working to turn out voters.

Betty Aldworth, the spokeswoman for the campaign behind the initiative, said volunteers would be making phone calls and would be especially active on college campuses. And she said organizations that have endorsed the initiative — the United Food and Commercial Workers union and the American Civil Liberties Union of Colorado, among others — would be reminding their members to vote.

“The only poll that counts is the one on Nov. 6,” Aldworth said. “We’re not taking anything for granted.”

Opponents, meanwhile, plan to increase the number of radio ads they’re running against the measure and highlight their heavyweight endorsers — including Gov. John Hickenlooper, U.S. Sen. Michael Bennet and Denver Mayor Michael Hancock.

But Roger Sherman, the manager of the opposition campaign, said it has been difficult even to buy time to run ads this season because of the campaign’s limited finances and the amount of other political advertising on the airwaves. Opponents have raised close to $650,000, according to their campaign filings. But that is only about a third of the $2 million that the main proponent committee has raised through contributions and loans.

Sherman said he believes the race is closer than The Post’s poll indicates but said opponents face an uphill battle in the campaign’s closing days.

“I think we’ve done what we can do, and it’s in the voters’ hands,” he said. “I remain cautiously optimistic that voters will do the right thing.”

The poll surveyed 695 likely voters from Oct. 28 to Wednesday on land lines and cellphones. Fifty-nine percent of people surveyed said they had already voted, while 18 percent said they would wait to vote on Election Day.

If voters pass Amendment 64, Colorado would be among the first — but perhaps not the only — states to legalize at least some marijuana possession for any purpose. Voters in Washington state and Oregon this year also are voting on marijuana measures, and polls show the initiative in Washington is likely to pass.

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold