PORTLAND – A referendum testing public sentiment for universal health care was approved by a narrow margin by city voters on Tuesday despite a lopsided campaign against the proposal.

The final unofficial tally showed the advisory referendum passing by 532 votes, or 52 percent to 48 percent. All told, 6,979 residents voted for the referendum and 6,447 residents voted against it.

About 300 supporters fanned across the city in the final month to overcome a television blitz launched by the opposition.

The tally “shows that it’s time to take a look at a plan that has worked in every other industrialized nation,” Jesse Graham, a community organizer for the Maine Peoples Alliance, said Tuesday night.

“Anyone that you talk to is upset with the health care system, whether it’s the high cost of prescription drugs or the fact that we have 165,000 people without insurance in Maine,” he said.

But the opposition said that the margin of victory was too narrow to send any clear message about universal health care.

“We went at this guns blazing because we didn’t want to send a mandate to Augusta that the Maine’s biggest city supports a single-payer universal health referendum,” said Daryn Demeritt, campaign coordinator for Citizens for Sensible Health Care Choices.

The costly campaign by the health insurance industry raised the stakes in a nonbinding vote on a resolution supporting creation of a single-payer system of health care in Maine’s largest city.

Supporters hoped to build support for universal health care as the debate on a Canadian-style single-payer system that provides comprehensive coverage shifts from the national to state level.

They were vastly outspent by opponents who contended that scrapping the current health care finance system would raise taxes and limit consumer choice.

Campaign finance reports filed last week with the city show that opponents who launched a TV blitz had raised more than $382,000 as of Oct. 25. Supporters collected less than $1,500 during the same period.