The fight over a ballot measure to help firefighters heats up in San Francisco.

But there’s a lot more to this one than meets the eye, affecting multiple layers of the City’s always complicated politics.

We are talking about Proposition H, on the November ballot, which would allow San Francisco firefighters to retire with their full pension at age 55 instead of 58. The firefighters’ union says too many of them are developing cancer from decades of on-the-job exposure, and allowing them to retire three years sooner will reduce that risk and save lives.

Sounds straightforward enough, and most voters will probably be sympathetic enough to a campaign featuring heroic firefighters getting cancer. But the city controller and Mayor London Breed say lowering the retirement age will cost the City many millions of dollars a year in additional pension costs, and that will balloon over time, and cash-strapped San Francisco just can’t afford that. So the firefighters union is going all out on this one, dropping its past support of Mayor Breed, and launching a $1.2 million dollar local ad buy today, which we were able to report exclusively on KCBS.