The little engine that could — the independent L.A.-to-Pasadena Gold Line construction agency — has battled larger interests in western Los Angeles for funding and sometimes for its very existence for decades.

Now, the San Gabriel Valley’s light-rail building entity is encountering resistance from the East.

A key bill that would allow the Metro Gold Line Foothill Extension Construction Authority to extend the rail line across the county border to Ontario International Airport is being opposed by one of its allies, the San Bernardino Associated Governments. The opposition threatens the vision to link downtown L.A. and Pasadena to an airport by rail, a first in Southern California.

Without passage of enabling legislation from Pomona Assembly Democrat Freddie Rodriguez, the authority will have to stop planning a leg to the airport.

“We need authority to do work beyond our jurisdiction,” said Habib Balian, chief executive officer with the Gold Line Authority.

SanBAG’s board voted earlier this month to oppose the bill, A.B. 2574, saying it jumps the gun by promoting a particular rail option to the airport before the agency has had a chance to study other options.

The San Bernardino County planning agency said it is looking at “30 alternatives which could connect transit to Ontario International Airport.” The agency also objected because it was not consulted and said it could impose “uncontrolled expenditures” on the agency.

The Los Angeles County Metropolitan Transportation Authority may also oppose the measure. Its board is scheduled to discuss the bill Thursday.

Rodriguez, despite opposition from San Bernardino cities, has not withdrawn the bill. It goes before the Assembly Transportation Committee on Monday.

“We want to keep the project moving forward,” said Rodriguez aide Francisco Estrada. “He believes in the vision of taking the Gold Line all the way to the airport.”

The Gold Line construction authority’s board is scheduled to vote on sponsorship of the bill on Wednesday. Its legislative committee recommended support of the Rodriguez bill.

The first extension of the foothill line is more than halfway completed. It will extend rail service from the Sierra Madre Villa station in east Pasadena to the second Azusa station at Citrus College. That extension will open at the end of 2015 or in early 2016.

A Phase 2b would continue east 12 miles through Glendora, La Verne, Pomona, San Dimas and Montclair. And a Phase 2c would continue through Upland, possibly stopping in Rancho Cucamonga before turning south and ending at Ontario Airport.

“There has been a long-held view that the connection to the airport would serve the San Gabriel Valley,” Balian said. He said a decline in Ontario Airport passengers recently was related to the recession and that the airport is and will remain viable.

Neither Phase 2b or 2c have any funding, Balian said.

But he said the Gold Line Construction authority has teamed up with SanBAG in the past to do a preliminary feasibility study that was released in 2008. Now, the Gold Line authority needs the OK from the state Legislature to work on a preliminary alternatives study, which would cost about $2 million, he said, and require funding from the airport.

The study will look at potential routes and environmental issues. “The foundation of the environmental work would start with this analysis,” he said.

Although SanBAG’s staff report says the bill is premature, Balian disagreed. He said planning for a light-rail extension can often take eight to 10 years before construction could begin.

“You want to start early enough,” he said.