If Rockefeller (D-W.Va.) decides not to return to the upper chamber, companies that ship bulk goods — primarily public utilities and industrial plants, including those in coal country — stand to lose. Big. And the freight railroads, which fight tooth and nail against shippers’ challenges to the rates they charge, stand to gain in equal amount.

Rockefeller has been a staunch advocate for so-called captive shippers, companies served by a single railroad that say they’re the victims of monopolistic practices by an industry almost entirely controlled by four railroads.

But the freight railroads are unapologetic about the rates they charge, saying they have to sink as much money as they can into keeping up with and improving their aging, far-flung infrastructure, which is a capital-intensive affair. They also counter that shippers can just use another method — such as trucks — if they don’t like their prices.

It’s an arcane, difficult, long-running fight between interests with constituencies and clout — not the sort of thing most lawmakers are eager to take on. But Rockefeller has not only waded into the fight, he’s come out swinging, saying in the past that the railroads are “grossly overcharging” and vowing that “railroad reform is going to happen.

He has repeatedly pushed for changes at the Surface Transportation Board, the agency charged with adjudicating rate disputes brought by shippers.

“I’ve been working on this for 26 years, so there’s a good deal of frustration in me,” Rockefeller said at a 2010 hearing on the subject.

Shelley Sahling-Zart, general counsel at Lincoln Electric System, said losing Rockefeller would be a big blow, not only because of his advocacy on behalf of the captive shippers’ interests but also because of the time it would require to bring someone else up to speed.

Sahling-Zart called Rockefeller a “champion” and a “leader” who has “years and years of institutional knowledge” who would be tough to replace on their issues. The other lawmaker often at the forefront of this fight, Sen. Herb Kohl (D-Wis.), has already announced his retirement.

Airports also are preparing for a possible Rockefeller departure, again with potentially large consequences — though it’s a more mixed bag, depending on the size of the airport.

For small, particularly rural airports, no longer having Rockefeller atop Commerce would be an enormous loss. But big airports might find something to cheer.

In the Senate, rural interests generally have a lot more juice than they do in the House. It’s been particularly important for rural airports to have a champion on top of the Commerce Committee who can fend off challenges to the Essential Air Service program, which was created as part of airline deregulation in 1978.

This small but controversial program, which regularly gets eyed for the budget ax, pays subsidies to airlines to keep them flying into small airports that otherwise wouldn’t be profitable enough to provide regularly scheduled service. Communities often believe having scheduled airline flights helps attract and maintain businesses, and so they fight to keep airline service. The program’s structure and funding was one of the flash points on the last FAA reauthorization, enacted earlier this year.

“That is going to be a target for Republicans going forward,” said one former committee aide. He noted that in Rockefeller’s absence, the lawmaker with the next biggest stake in the Essential Air Service program is Sen. Mark Begich (D-Alaska), who isn’t senior enough to exert much leverage on the subject.

On the flip side, larger airports may benefit if Rockefeller leaves, because he has staunchly opposed one of the marquee requests the airport lobby had as part of the FAA bill — raising the current cap on per-segment flight fees, called the Passenger Facility Charge. PFCs are now capped at $4.50 per flight segment. Many airports aren’t yet charging the maximum PFC, but airports that are maxed out are almost always large ones.

Rockefeller’s view, according to one airport source, was that people flying into and out of small airports typically have to travel through hubs to get almost anywhere. The more flights a person has to take, the more they’ll pay in PFCs.

That dynamic could change especially dramatically if Rockefeller is replaced by a Democrat from a more urbanized state. The next most senior person on Commerce would be Sen. John Kerry (D-Mass.), whose state includes Boston Logan International Airport — although Kerry, regularly discussed as a potential second-term Cabinet member for President Barack Obama, may not be a senator much longer.

Next in the seniority list is Sen. Barbara Boxer (D-Calif.), who has several large airports in her state.

But Boxer, who has made her bones as a progressive and environmentalist, would have to give up her chairmanship of the Senate Environment and Public Works Committee to take the reins at Commerce. And she told POLITICO through a spokeswoman that she won’t make the leap, saying she “has yet to finish her work” on the environment committee.

Some observers had speculated that Boxer might switch gavels because of the enormous swath of jurisdiction Commerce claims, which includes technology and telecom issues — as well as the chance it would afford the consumer-minded senator to closely oversee matters of consumer rights and safety for transportation.

With Boxer taking herself out of the running, the domino then falls toward Sen. Bill Nelson (D-Fla.), whose public role on Commerce has mostly been as an advocate for spaceflight programs and jobs at Kennedy Space Center. However, given how important tourism is to Florida, Nelson would probably be a reliable voice in favor of the airline industry.

“Very few people are getting to Florida by bus — and airlines are a huge part of that equation,” said an airline lobbyist.