Airbnb has spent more than $8 million and hired a top political operative to defeat a San Francisco initiative on the ballot Tuesday that could threaten the growth of one of the most valuable global technology companies. Proposition F, which would limit short-term rentals, was brought by affordable housing advocates fed up with the city’s housing stock being used as rentals for tourists while residents face skyrocketing rents and evictions. For Airbnb, a defeat in its hometown of San Francisco would be mostly a symbolic blow. Should similar measures be introduced elsewhere, however, the company could face serious financial consequences. At stake is its ability to continue adding rentals at the same speed, increase revenue and maintain its $25.5 billion valuation, all of which fall under greater scrutiny as it moves closer to an initial public offering.