An advertising blitz funded by tobacco companies has eroded Californians' support for a ballot measure to raise taxes on cigarettes, putting the vote's outcome in doubt.

Philip Morris and RJ Reynolds have flooded airwaves with warnings that the proposed $1 tax on cigarette packs is a flawed idea which would bloat government bureaucracy and funnel money out of the state.

The energetic $47.7m campaign – more than triple the yes campaign – has been fronted by anti-tax activists and dramatically reduced support for Proposition 29, a June 5 ballot measure backed by anti-cancer groups.

"We are still ahead but it's very close. Big tobacco has a bottomless budget to tell lies," said David Veneziano, head of the American Cancer Society's California chapter. "They are trying to protect their profits."

Two months ago about two-thirds of voters backed the measure but that has tumbled to just over half, according to a Public Policy Institute of California survey. "Today, 53% say they will vote yes, 42% say they will vote no, and 5% are undecided on the measure."

A coalition of pro-business and anti-tax groups has warned voters that the tax would benefit health industry lobbyists and do little for people with cancer. "You have to step back from the emotional appeal and look at the big picture here," said David Spady, of Americans for Prosperity. "You're going to have a nine-member unelected board that will determine how tax dollars are spent."

The no campaign said the tax revenue would duplicate existing programmes and be spent outside the state. It ran an advert in which a real doctor, La Donna Porter, wearing a white coat, urged viewers to vote no, saying needless bureaucracy would result. Porter also fronted a ad for the chemical industry in 2002. She disappeared from screens last month after questions over her relationship with the tobacco

industry risked damaging the no campaign.

Philip Morris and RJ Reynolds have been largely invisible but according to the watchdog group maplight.com they have bankrolled almost the entire no campaign. Porter, and groups fronting the campaign, have denied receiving tobacco funding. "We are independent. We're not beholden to anyone," said Spady, of Americans for Prosperity.

Stanton Glantz, a prominent anti-smoking activist and director of the Center for Tobacco Control Research and Education at the University of California, scorned such claims, saying there were multiple ways for tobacco companies to mask payments. "They do their best to hide."

Philip Morris and RJ Reynolds referred Guardian queries about funding to an umbrella group, Californians Against Out-of-Control Taxes and Spending. It declined to provide figures but said funding was legal and transparent.

"These contributions can be found online at the Secretary of State's website at www.sos.ca.gov in its Cal-Access system," said Beth Miller, a spokesperson. The no campaign was a broad-based coalition, she said. "We all believe cancer research is important, but California can't afford to start a new billion-dollar spending programme when we have a $16+ billion budget deficit."

Under proposition 29, the tax per pack would jump from 87 cents to $1.87.

The initiative's sponsors – the Cancer Society, the American Lung

Association and the American Heart Foundation – say it would raise

more than $750m for cancer research and stop 220,000 children from

taking up smoking.

"Within five years the idea of smoking as a socially tolerated behaviour could simply collapse," said Glantz. "This is a tremendously important fight. It could make California the first state where where the tobacco epidemic has been ended."

Activists hope success will embolden other states to increase cigarette taxes. They estimate tobacco companies would lose about $1bn in revenue per year in California alone. The yes side has mustered $12.2m, including a $1.5m donation from the Lance Armstrong Foundation, to fight back with its own television campaign in which characters satirically endorse the other side.

"I support big tobacco because I like their ads," smiles one mother. "And so do my kids." A farmer in a field says: "I support big tobacco because they killed my wife. And that's one less mouth to feed." A cyclist adds: "I support big tobacco because spending $9.14bn on tobacco-related healthcare costs is exactly what California needs right now."