A sales-tax measure on the March ballot could be a do-or-die vote for Sonoma-Marin Area Rail Transit, the charming but struggling commuter rail that links North Bay residents to jobs in San Francisco.

As the clock ticks down to March 3, big players have begun to weigh in. This week a string of anti-SMART ads popped up on local television and radio, decrying the 30-year sales-tax extension — called Measure I — as a blank check with no accountability.

Sponsored by a committee called NotSoSmart.org, the ads feature evocative images of cars stopped at empty railroad crossings and checks made out to the “SMART Bureaucracy.” Molly Flater, the daughter of the influential Sonoma County developer Bill Gallaher, is funding the effort.

So far, Flater has chipped in $559,183 to defeat Measure I, according to Mike Arnold, a committee treasurer. The committee has not yet made its financial disclosures public.

Flater is chief operating officer of Gallaher Homes, a family-owned real estate firm with developments throughout Sonoma County, including luxury apartments, town houses and a shopping center. Its sister division, Oakmont Senior Living, includes more than 40 retirement communities on the West Coast.

“If I end up spending $1 million to save our community taxpayers from a $2.4 billion mistake, then I feel it is worth every penny,” Flater wrote in a letter she has circulated to reporters and others. That’s her estimate for the total revenue the tax extension will generate. Officials estimate the quarter-cent tax will produce $40 million a year.

Without Flater’s backing, the anti-SMART bloc seemed outnumbered by the politicians and commuters who want functioning public transportation in the North Bay. Arnold is among the dissenters who criticize the train as a waste of taxpayer money.

“That’s my primary concern,” he said, noting that SMART, which carries 2,700 passengers on weekdays, is sparsely filled when compared to San Francisco’s Muni system, which packs about 720,000 weekday riders.

Flater laid out her arguments in the letter, upbraiding the rail system for failing to meet the promises of its original tax-measure campaign in 2008: a 71-mile railroad from Larkspur to Cloverdale, with a 71-mile bike path snaking alongside it and shuttle service to and from stations. Those plans stumbled as construction costs soared and tax revenue dropped significantly during the recession. To date, SMART has only built 45 miles of rail and 23 miles of bike path.

It’s not the first time members of the family have sparred in local politics.

Flater’s husband, Scott Flater, and father sued the Santa Rosa Press Democrat for defamation after the newspaper documented Scott Flater’s independent expenditures to support three Santa Rosa City Council candidates. The two plaintiffs lost their case last year.

Sales taxes are the lifeblood of Bay Area public transportation systems, and the quarter-cent tax that Marin and Sonoma county voters approved in 2008 is critical for SMART. It’s set to expire in 2029, and if voters choose not to extend it, then officials will have to eat through the rail system’s reserve funds.

With debt payments mounting annually, SMART would deplete its reserves by 2024 if the system continues to spend at the current rate and the tax is not renewed, said SMART board chair Eric Lucan. A 30-year extension would allow the transit agency to restructure the debt, freeing up $12.2 million annually that could go back to operations.

“This is critical,” Lucan said. He emphasized the growing popularity of the rail line, which recently opened stations in Larkspur and downtown Novato and now runs trains every 32 minutes during peak commute hours.

It’s beloved by riders who sought relief from the choked traffic on Highway 101. On the SMART train, they could plug in their laptops and start work early, or gaze out the window at a landscape of wetlands and grassy hills. Some cheekily applaud when the train whooshes by a particularly painful stretch of 101 called the Narrows, where the freeway shrinks from three lanes to two.

“For me, it was just a great way to start and end the day, particularly when you’re cruising past standstill traffic both ways,” said Mike Grant, who rode the train for 14 months from his home in Petaluma to work at the Marin County Office of Education in San Rafael.

Lucan is hopeful. An outside poll by the North Bay Leadership Council showed that 69% of respondents favored Measure I, surmounting the two-thirds threshold for approval.

But the sales-tax extension is by no means a done deal. Voters rejected the tax before — the SMART ballot measure lost in 2006 before passing in 2008. And this year it will wind up on ballots alongside parcel taxes for schools and a sales tax for fire prevention in Sonoma County. When voters get intimated by a tax-heavy ballot, they often vote “no.”

A well-funded campaign by a rich family could be the biggest threat, according to Lucan.

“That’s something that could change the dynamics of this campaign,” he said. “I think the people of Marin and Sonoma County need to wonder why there’s influence in this election by a multimillionaire family to take away transit options for seniors, disabled people and working families.”

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com Twitter: @rachelswan