The measure would ask voters across five Bay Area counties to approve the tax, with proceeds directed at preventing service cuts and averting what backers have described as an austerity scenario — or, in the worst case, a shutdown of BART operations altogether. The signature submission marks the end of the qualifying phase and the start of a campaign that will play out regionwide.

The stakes for San Francisco riders are significant. SFMTA has leaned on BART as a backbone of regional commute capacity, and any reduction in BART service would push additional load onto a Muni system already running a structural deficit. Neither agency has released modeling of what a failed measure would mean for combined system capacity.

What the sales tax rate would be, how revenue would be split among the five counties, and what oversight structure would govern spending have not been detailed in public campaign materials reviewed for this story.

The measure still needs a formal certification of the signature count before ballot language is finalized. Voters will then weigh it alongside whatever else lands on the November ballot — a crowded field in a presidential election year.

Watch for: certification of the signature count by county election officials, release of the official ballot measure language, and the first independent fiscal analysis from the city controller's office. The real fight over messaging and county-by-county coalition building starts now.