Toward the bottom of Tuesday’s ballot — past the presidential candidates, the U.S. Senate race and the long list of statewide propositions — is a half-cent sales tax proposal that would fund the most ambitious transit expansion in Los Angeles County history.

Measure M would generate an estimated $120 billion over four decades for Metropolitan Transportation Authority bus and rail operations, system maintenance and fare subsidies, as well as a dramatic expansion of the county’s growing rail network.

The proposal has the potential to transform a traffic-choked region that began building a modern rail network decades after other U.S. cities. The tax revenue would fund the construction of nearly a dozen new rail lines, including twin rail tunnels through the Sepulveda Pass and new connections to Pacoima, Claremont, Artesia and Torrance.

Tax revenue would also fully or partly fund 10 highway projects, including several new tolled carpool lanes, an extension of State Route 71 and a new carpool-lane interchange between the 405 and 110 freeways.

Measure M would raise the county’s base sales tax rate by a half-cent in 2017 and increase it to 1% in 2039 after another half-cent tax expires. The levy would continue until voters choose to end it.

Traffic always ranks among the top priorities for Los Angeles residents, often outstripping concerns about the economy, crime and retirement savings. But the two-thirds majority required to pass such a tax is always a high hurdle.

Measure M is listed alongside a host of other spending proposals, including a county parcel tax for parks and a community college bond measure. In the city of Los Angeles, voters will decide Proposition HHH, a $1.2-billion bond measure for homeless housing.

Metro’s chief executive officer, Phil Washington, has said he hopes to convert 20% to 25% of the county’s population into regular transit riders, more than three times the current rate.

Because Measure M does not have an end date, critics have said, voters may find it difficult to hold Metro accountable for cost overruns and schedule delays. Metro has grappled with those issues in the recent past, including on the 405 Freeway widening, which was years late and hundreds of millions of dollars over budget.

Measure M has faced opposition from mayors and council members in the South Bay and a swath of cities in the southeast county, who have said its project timeline favors the northern half of the county.

Critics have noted that, despite the popularity of the new Expo Line extension to Santa Monica, Metro’s ridership has faltered. Trips on the region’s sprawling bus and rail network have fallen nearly 10% over a two-year period, with bus trips down by almost 13%.

Should Measure M fail, Metro will continue to receive more than $2.5 billion annually from three other dedicated half-cent taxes. But without a new, permanent source of revenue, officials say, they won’t have the borrowing capacity to dramatically expand the rail network.

The most visible advocate for Measure M has been Los Angeles Mayor Eric Garcetti, who has traveled across the county for news conferences, and appeared in a series of televisions ads funded by the Yes on M campaign.

In one advertisement, filmed as Garcetti is driving, he calls Measure M “something that will help relieve traffic and literally change our lives.”

The Yes on M campaign has raised nearly $10 million in campaign contributions, with strong backing from developers, labor unions and construction firms, as well as several philanthropists, including Eli Broad and Jerry Perenchio.