Artist rendering of the interior of planned Purple Line commuter trains in Maryland. (Purple Line Transit Partners)

Metro’s mounting maintenance and safety problems have provided a fresh legal argument for opponents of the Purple Line, who say Maryland can’t afford to build a new light-rail line when the subway system it will connect to needs millions in repairs.

In an ongoing federal lawsuit, opponents also argue that Maryland officials can’t prove the Washington region needs another rail line when Metro has steadily lost riders over the past six years. Riders transferring to and from Metro are projected to make up about one-third of the ridership of the 16.2-mile Purple Line in the Washington suburbs.

The Maryland Transit Administration (MTA), which plans to begin the line’s construction this year, justified the project’s need and cost based largely on its connection to a “thriving Metro system,” the opponents say. ­Metro’s severe deterioration, they argue, provides “new circumstances or information” that requires additional environmental review, including more consideration of less expensive transit options.

The opponents also argue that federal law requires that a local transportation network have enough funding to operate and be maintained before federal money is used to expand it.

“We have a dire need to fix Metro before we attach things to it that rely on it,” said John M. Fitzgerald, a Chevy Chase resident and environmental lawyer who is one of three plaintiffs in the 2014 lawsuit. The Purple Line “is such an expensive project and Metro is in such dire need that it makes common sense to stop and take a second look. Luckily, it’s in the law.”

After years of neglect that has led to critical safety lapses, electrical fires and almost daily breakdowns, Metro plans to begin a nearly year-long maintenance blitz Saturday.

The Purple Line, which will connect Montgomery and Prince George’s counties, will not officially be part of Metro; it will be owned and operated by the MTA. Its light-rail trains will be shorter and slower than Metro’s.

Even so, the two systems will be inextricably linked, both in their ridership and revenue.

Maryland officials have long promoted the Purple Line as a much-needed east-west transit link between the state’s spokes of the Metro system. The four Metro stations along the Purple Line alignment — Bethesda and Silver Spring on the Red Line, College Park on the Green Line and New Carrollton on the Orange Line — are projected to be the light-rail system’s busiest.

Maryland Transportation Secretary Pete K. Rahn declined to comment on the lawsuit but said, in general, that he has no concerns about the state’s financial ability to build the Purple Line and help fix Metro simultaneously.

While Maryland will pay $160 million during the Purple Line’s projected six years of construction, most of the line’s $2 billion or so in building costs will be paid by federal aid, county contributions and private financing via a public-private partnership, Rahn said.

The debt service that the state will have to pay on the private financing won’t kick in until the line opens to passengers in 2022, he said. By then, Rahn said, ­Metro’s repairs should be completed — and the riders who fled because of unreliable service will have returned.

“I think five years, even on the outside, is a reasonable time for them to accomplish what they need to do,” Rahn said of Metro. “I believe the issues at Metro will be well resolved by the time we [open] the Purple Line, so I don’t believe they’ll have any impact.”

Rahn said Metro General Manager Paul J. Wiedefeld hasn’t told Maryland how much it and other Washington-area jurisdictions will be asked to contribute toward Metro’s rehabilitation. But Rahn said he expects that any amount beyond the $400 million to $500 million Maryland currently contributes annually to the transit system will be stretched out over several years — timing that he said the state’s nearly $5 billion annual transportation budget can accommodate.

Fitzgerald, Chevy Chase resident Christine Real de Azua and the advocacy group Friends of the Capital Crescent Trail filed the federal lawsuit in 2014 against the Federal Transit Administration, the U.S. Fish and Wildlife Service and the Transportation and Interior departments. The MTA became a co-defendant in 2015.

Attorneys for both the state and federal agencies have responded that Metro’s problems “do not involve the Purple Line, and they have no relationship to the environmental impacts of the Purple Line,” according to legal briefs. Metro’s shortcomings, they say, necessitate no additional environmental review of the Purple Line.

But Metro’s falling ridership has prompted hand-wringing among region leaders concerned about its fare revenue. After peaking at 225 million passenger trips in 2009, ridership began to fall and hasn’t stopped. In the fiscal year that ended last June, the system logged 206 million trips, and the downward trend has continued this fiscal year.

State lawyers argue that ­Metro’s declining ridership has no bearing on Purple Line ridership forecasts that were made as part of the environmental analysis approved in 2013. The study compared the costs, benefits and impacts of a new light-rail line with other transit proposals, such as an express bus line. “A decrease in Metro passengers would affect all alternatives equally,” the state’s lawyers wrote.

Maryland officials are concerned with ridership numbers on Metro and, eventually, the Purple Line because the state is counting on Purple Line fare revenue to pay the debt service on the private construction financing. Any financing costs that the Purple Line fares don’t cover will come from fare revenue collected on the state’s commuter rail system, Maryland officials have said. Advocates for the commuter line, MARC, say that system also needs money to expand.

In recent court filings, Fitz­gerald and the other plaintiffs also argue that the Purple Line warrants further review to consider the environmental impacts of recent cost-cutting changes. The lawsuit also alleges that the state didn’t adequately study the impact of the rail line on noise, migratory birds and a tiny, endangered crustacean previously found downstream of the Purple Line alignment.

Fitzgerald said the lawsuit raised Metro’s meltdown as an issue early on, but he said the plaintiffs began to focus on it more last fall, after the National Transportation Safety Board deemed Metro’s safety problems to be so severe and persistent that the system required stronger federal oversight.

“That was a big signal saying, ‘Wait a minute, this is really serious,’ ” Fitzgerald said.

Ralph Bennett, president of the advocacy group Purple Line Now, called Fitzgerald’s arguments “absurd,” saying Metro’s maintenance and safety problems will be fixed well before the Purple Line opens.

“The reason it [Metro ridership] is falling is because service is being compromised,” Bennett said. “When service resumes to the state that everyone expects, the ridership will be back in force, and the need for a comprehensive transit system — of which the Purple Line is an important part — will be more evident than ever.”

U.S. District Judge Richard J. Leon is expected to hear oral arguments in the case this summer.