Now that the fracas over the payroll tax cut appears to be winding down, focus is shifting toward the controversial transportation bill bobbing its way through the House. It’s a messy bill with a lot of moving parts, so here’s an overview of where things stand.

What’s all the fuss about? (David Paul Morris/Bloomberg)

First, there’s the drilling component. As we’ve seen, there’s not nearly enough gas-tax revenue these days to maintain current levels of highway and transit spending. So, rather than simply hike the gas tax, Republicans in the House are mulling various proposals to make up the $60 billion shortfall. The popular option? Revenues from new drilling in places like the Arctic National Wildlife Refuge, coastal regions, and oil-shale leases. Environmentalists loathe this idea. (They also note that oil shale — not to be confused with shale oil — isn’t even commercially viable yet.) Conservative groups, too, are grumbling that this bill would essentially spend money based on revenues that don’t even exist yet, and may not materialize for years.

Then there’s the transit controversy. Since the days of Ronald Reagan, mass transit has been guaranteed a one-fifth share of gas taxes and other fees in the Highway Trust Fund. But under an amendment recently passed by the House Ways and Means Committee, mass transit would no longer receive this money automatically — instead, subways and bus systems would have to compete with a bunch of other programs for the $40 billion in transit funds. The measure’s Republican backers have said that they’re not “anti-transit,” they just want local governments to fund transit with user fees instead of gas taxes. They also argue that this bill gives states more flexibility to decide what projects to build.

Transit advocates counter that states already have this flexibility. Instead, what will happen is that mass-transit projects could get crowded out by other priorities in the budgeting process. “Without the certainty that gas tax revenues provide,” warn Streetsblog’s Ben Fried and Ben Goldman, “transit agencies will immediately move to cut service and raise fares.” For another, subtle critique, see Alon Levy, who notes that transit projects do deserve closer scrutiny, but the House bill seems to just defund public transportation without more accountability. Indeed, one scenario is that worthy projects could get crowded out by smaller, cheaper transit ideas that are ultimately less cost-effective.

So how will this play out? It’s still possible that the transit provision won’t survive the House. A number of Northeastern Republicans — particularly those from New York — are teaming up with Rep. Jerry Nadler (D-N.Y.) to back an amendment to restore gas-tax revenues to mass transit. “This will be a big test in many ways,” says David Goldberg of Transportation for America. “The bill has a long road to 218 votes [the majority needed for passage] if this transit provision remains.”

For his part, House Speaker John Boehner is trying to diffuse the dissenters by splitting up the House bill into multiple pieces — the energy provisions that deal with drilling will be voted on separately from the transportation parts. Boehner says that this allows each major component to “be debated and amended more openly.” But it also divides the rioters. Republicans opposed to drilling in Alaska can’t join hands with the Republicans opposed to the transit provisions and scuttle the entire bill together. We’ll see if this gambit works.

To make matters even more complicated, the House bill will still have to be reconciled with the Senate’s own bipartisan transportation bill, which is still getting pieced together but looks very different. Plus, the House may add yet more volatile amendments (Boehner has promised a vote on the Keystone XL pipeline, triggering a White House veto threat). Plus, there are no earmarks in the bill, which makes passage all the more difficult. This is going to get rowdy.