Los Angeles County voters decisively passed Measure M, the half-cent sales tax increase to fund billions of dollars in transportation projects over four decades.

Now that the country has elected Donald Trump as its president, and a Republican-dominated Congress, how might this impact the Metropolitan Transportation Authority's push to build out the county transit system?

First, think of the county transit agency’s budget as a pie. Local sales taxes like Measure M, which is the fourth such tax increase since 1980, make up almost a two-thirds slice of Metro's budget. This puts Metro in good shape, according to Yonah Freemark, an urbanist who writes about transit issues.

"The good news is Los Angeles has plenty of money to play with transportation as it wishes," he said.

But he warned that one of the smaller pie pieces, federal funding, isn't guaranteed.

"The ambitions of Measure M, the things that voters were promised, do rely on those federal funds," he said.

Loss of anticipated federal matching funds for some Measure M projects could force Metro to lengthen its timeline for construction — unless it can find alternative sources of funding, like a private partnership or state money.

Federal funding used to make up between 12 and 19 percent of Metro’s budget. It's an even larger part of major projects like the ongoing Regional Connector and the Purple Line subway.

But federal funding has been shrinking over the years, as the National Highway Trust Fund, from which its drawn, has been reduced due to dropping gas tax revenues.

The Republicans’ party platform calls for ending transit funding completely, although current commitments under the FAST Act transportation bill passed last December won’t expire until 2020.

Freemark said Congress could conceivably fiddle with the transit allocation numbers when it approves yearly budgets but he doesn't expect this to be a priority.

The new federal government could also change course when it comes to allocating discretionary grants, such as Transportation Investment Generating Economic Recovery, or TIGER grants.

These are smaller, project-specific grants awarded by the federal Department of Transportation. Los Angeles has won three grants totaling $35 million over the last two years for bike and pedestrian projects.

Under the new administration, it's less likely urban transit-oriented projects will be rewarded as they were under the Obama administration, said Freemark.

President-elect Trump’s trillion dollar infrastructure proposal to build bridges, roads and other projects could prove a boon for Los Angeles, Freemark said, if it includes transit.

But Martin Wachs, UCLA professor emeritus of urban planning, is skeptical the plan will help transit because initial reports indicate the proposal would be built on private financing.

"With public transit, that’s harder to do because the operation of a public transit system involves losses rather than gains. The fares that people pay don’t cover the operating costs," he said.

He said Trump’s plan, as currently laid out, would favor projects like toll roads that can generate a profit for investors.

Trump also has to convince Congress, including members of his own party, to approve his plan. Last week, Senate Majority Leader Mitch McConnell said the infrastructure plan is not a top priority.