Maryland Gov. Martin O’Malley (D) has long promised that the Washington suburbs and Baltimore would each get a new light-rail line and that the Purple Line and its counterpart in Charm City could be built at the same time.

But state financial documents recently submitted to the Federal Transit Administration show that O’Malley’s promise, to the state’s most populous regions, will be difficult — if not impossible — to keep.

The General Assembly’s recent rejection of the governor’s proposed gas tax hike makes it increasingly likely that the state will have to choose to build one of the lines before the other, state and local transportation officials say. With no new tax revenue dedicated to transportation, finding the money for even one of the light-rail lines will be difficult, the officials say.

The state hoped to begin construction on both lines in 2015, with the 16-mile Purple Line opening between Bethesda and New Carrollton by late 2020 and the 14-mile Baltimore Red Line opening in early 2021. But construction of the runner-up project probably would be delayed at least five years, until after the chosen line is built.

When asked whether the state could afford to build both at once without a gas tax increase, state Transportation Secretary Beverley Swaim-Staley said, “The short answer is probably no. . . . Obviously, it would have a significant impact on our ability to build anything else.”

Time to find additional money is short. State and local officials say they have 12 to 18 months to prove to the Federal Transit Administration that Maryland can pay half the cost of constructing one or both lines. Otherwise, the state could fall behind in the rigorous competition for federal transit money.

“Obviously, we see it as a setback,” said Ralph Bennett, president of the advocacy group Purple Line Now, referring to the gas tax proposal’s demise. “The federal government will look to us and say, ‘How’s your funding coming?’ Now we have to tell them we’re working hard in Maryland to get reliable transportation funding, but we only got as far as we got.”

Swaim-Staley said building both projects at once is still “technically affordable,” even without a gas tax increase. But based on projected revenue, doing so would consume 46 percent of the state’s transportation construction funds and 11 percent of its entire transportation trust fund through fiscal 2020, according to the state reports.

The state relies on the transportation trust fund to maintain much of its infrastructure, including highways, bridges, airports and the busy port of Baltimore. The fund also pays for Maryland’s contribution to the Metrorail system, which opened in 1976 and is struggling to maintain its infrastructure. A state commission recently reported that without new revenue, Maryland’s transportation system is “on the verge of financial collapse.”

Officials estimate that it will cost $1.93 billion to build the Purple Line and $2.2 billion to build Baltimore’s Red Line, an east-west addition to the city’s light-rail system.

The state gas tax was last increased in 1992, but opponents have said that Marylanders still reeling from the recession are already facing other increases in taxes and environmental fees. As gas prices soared to more than $4 per gallon in the legislative session, the idea of taxing more at the pump never gained traction.

State Senate President Thomas V. Mike Miller Jr. (D-Calvert) said this week that he still hopes the General Assembly can address transportation funding needs this year. The legislature could convene again in 2012 for a special session in the summer to consider expanding casino gambling.

Swaim-Staley said the state is exploring other ways to help fund the rail lines’ construction. Those include the potential for public-private partnerships and financing bonds through fees or special taxes on developers and landowners whose property values would increase because of their proximity to stations.

“It will certainly require more creativity than the old system, where you could just go to the transportation trust fund,” said Swaim-Staley, who announced last month that she is stepping down July 1.

Adding to the uncertainty is continued debate in Congress over long-term federal transportation funding, including how much to allot to new transit. Maryland is seeking federal money for half of the construction costs for each light-rail line.

To begin building on schedule in 2015, both projects would need to enter the “final design” phase in the federal funding process next year. But first, the state would have to “commit” at least half of its share.

“At some point we’ll have to demonstrate that we have the cash,” Swaim-Staley said.

Transit experts say a Purple Line, like the Metrorail Silver Line extension being built in Northern Virginia, would probably need a mix of state, federal, local and private funding.

But Purple Line supporters say they are concerned about relying on taxing commercial property owners to the extent the Silver Line project has. While the economic opportunities in Tysons Corner and along the Dulles Corridor are not in doubt, some of the older Maryland suburbs along the Purple Line’s proposed route have struggled to attract development.

This has been especially true in Prince George’s County, even in areas near Metrorail stations. In Montgomery County, most areas around prospective Purple Line stations, including in Bethesda and Silver Spring, are either heavily residential or mostly built out.

Money aside, the project also faces significant opposition — and potential legal delays — from the town of Chevy Chase, a country club and trail advocates who say a rail line would destroy the wooded Georgetown Branch walking and biking path between Bethesda and Silver Spring.

Purple Line supporters say that next year would probably be the last chance for a gas tax increase before both projects faced delays. A major tax hike in 2014 — an election year — is highly unlikely. By the 2015 legislative session, the schedule for both projects would have slipped at least two years.

Del. Tawanna P. Gaines (D-Prince George’s), chair of the House appropriations transportation subcommittee and a Purple Line advocate, said she’s “very hopeful” a gas tax could pass before the 2014 elections.

“I don’t think anyone was thinking about the transit lines” when the proposal failed this session, Gaines said. “Now we have additional time to focus on transit lines. Hopefully, people will understand the importance of transit in the state.”

Staff writer Aaron C. Davis contributed to this report.