PORTLAND — A late surge in voting on Tuesday propelled Question 1 to a narrow victory, 4,888 to 4,605, and at least temporarily blocked the sale of Congress Square Park to developers of the Westin Portland Harborview Hotel.

The margin of support for the referendum, 51 percent to 49 percent, reflected about an 18 percent turnout by the city’s 52,330 registered voters.

“It was very close, but we were happy to win,” said Protect Portland Parks co-chairman David LaCasse on Tuesday evening, after the unofficial results were in. “It highlighted how important parks were to people.”

Passage of the referendum adds 35 parcels to the city Land Bank, established in 1999 to preserve public space, and mandates approval by eight city councilors before a Land Bank parcel can be sold.

Included in the amendments to the city Land Bank ordinance is the entirety of Congress Square Park. Last September, the City Council voted 6-3 to sell about 9,500 square feet of the plaza at Congress and High streets to the Westin developer, RockBridge Capital.

The referendum requires a citywide vote should seven or fewer councilors approve the sale of a Land Bank parcel, and is retroactive to Sept. 6, 2013, shortly before the council approved the Congress Square sale.

Opponents of the referendum, led by former Mayor Jim Cohen and the political action committee Forward Portland, had raised more than $53,000 to defeat the referendum.

“Obviously, we are disappointed in the vote,” Cohen said Tuesday night. “But we do believe people want the square improved.”

Next steps remained elusive for both sides. City Corporation Counsel Danielle West-Chuhta has said the council’s approval of the sale could trigger another referendum, but Cohen was not sure what members of Forward Portland might do next in support of the sale.

Rob Levin, the attorney who wrote the referendum’s amendments to the ordinance, said he hoped the vote would renew discussions of park improvements, including suggestions already under consideration since last summer.

“We are looking forward to working cooperatively,” Levin said. “We hope to re-start the conversation. There is lots of passion on both sides.”

The victory at the polls for Protect Portland Parks came after a long struggle with the city to even get the referendum on the ballot. City officials refused to recognize the petition drive for the ordinance amendments, saying they covered administrative aspects that were the responsibility of city councilors.

Last October, Cumberland County Superior Court Justice Joyce Wheeler ruled in favor of Protect Portland Parks and allowed the petition drive to go forward. The city appealed her decision to the Maine Supreme Judicial Court. Maine’s high court upheld Wheeler’s decision on May 6, allowing the referendum to occur Tuesday.

Councilors also enacted a separate measure designed to protect many of the same parcels, but excluded the 9,500 square feet of land to be sold to RockBridge. The developer has planned to use the space to add an events center to the Westin.

The amendments to the city Parks, Recreation and Public Building Code require approval by seven councilors to sell land after gaining a favorable opinion from the city Parks Commission. The amendments went into effect May 28.

Forward Portland members included Mayor Michael Brennan, Councilors Ed Suslovic and Jon Hinck, former City Manager Jim Gray and State Theatre and Port City Music Hall General Manager Lauren Wayne.

Spending reports filed by Protect Portland Parks and Forward Portland through June 6 show each side received infusions of cash in the final days leading to the election. On June 4, RB Portland LLC, the RockBridge entity listed as buyers of the park land, contributed $12,500 to Forward Portland.

In all, RB Portland contributed $20,000 of the more than $53,000 Forward Portland  raised. The Portland Regional Chamber of Commerce contributed a total of $15,000, with $10,000 listed as from “commercial sources” and the remainder from a “non-profit organization.”

Protect Portland Parks received a $10,000 contribution from Diversified Communications on June 4 and a $10,000 loan from Neal Street resident Noni Ames. The campaign received $30,000 total in loans, with two other $10,000 loans coming from Alison Hildreth and LaCasse.

On Tuesday, Levin said he hoped the loans could be repaid after the city pays legal costs associated with contesting the referendum question.

David Harry can be reached at 781-3661 ext. 110 or dharry@theforecaster.net. Follow him on Twitter: @DavidHarry8.