The Washington area's long-debated plans to build the Green Line and other parts of the proposed 103-mile Metro subway system again have become entangled in a political battle, jeopardizing prospects for the system's completion and testing the willpower, financial resources and ingenuity of federal and local officials.

Less than a year ago, many of the system's problems appeared to have been solved. Metro, District and Prince George's County officials had overcome a series of conflicts, including a federal court contest, to clear the way to construct the Green Line, Metro's only unopened route.

After long negotiations, city and county governments had drawn up a plan to expand the rail system to 89.5 miles by building extensions in Prince George's and Northern Virginia. The Reagan administration had lifted a controversial 76.4-mile limit on subway construction and tentatively approved the 89.5-mile plan.

Now, the financial underpinnings of Metro's plan have collapsed and the outlook for the system is unclear. The Reagan administration, in a turnabout, has moved to cut off federal spending for Metro construction, citing pressures of the Gramm-Rudman-Hollings deficit reduction law.

Without further congressional appropriations, officials say, the 89.5-mile plan almost certainly will be delayed and may never be completed.

Four subway extensions are at issue. These include the central section of the Green Line, connecting Anacostia with a U Street station in Northwest Washington, and a Red Line extension to Wheaton in Montgomery County. Construction of these segments is well under way.

Also in jeopardy are a Green Line branch linking the Fort Totten station in Northeast Washington with Greenbelt in Prince George's and a Yellow Line spur to Alexandria's West End. Other parts of the proposed 103-mile system remain unfunded, but officials had not expected to start work there for several years.

Today the Metro system encompasses60.5 miles, and its size will increase to almost70 miles when the Orange Line is extended to its Vienna terminus in Fairfax County next June. Officials say that the Vienna extension is not endangered by the current crisis because funds already have been set aside.

This week, Ralph L. Stanley, the administration's mass transit chief, spelled out two options under which Metro construction could move ahead without further congressional aid.

Under one proposal, Metro could revise its plans and put all its money into completing the Green Line between Anacostia and U Street, along with the Red Line extension to Wheaton. These segments could be financed with about $400 million in congressional aid, which Stanley previously has refused to release.

Such a move would, in effect, reinstitute the 76.4-mile plan and raise what some officials view as virtually insurmountable political obstacles because of its impact on Prince George's and Northern Virginia. Metro would have to take funds away from routes in those areas to finance stations in the District and Montgomery.

Prince George's officials, who complain of getting short shrift in Metro's construction schedule, oppose the proposal. A spokesman for County Executive Parris Glendening termed Stanley's suggestion "incredibly presumptuous."

Said one transit analyst: "Politically, I don't know how they can back away from P.G. County."

The second option outlined by Stanley would allow Metro to continue work on its 89.5-mile plan with additional state, county, city or private funds. Metro would need to obtain guarantees from Maryland, Virginia and the District that any projects begun with federal funds could be finished with nonfederal funds.

Officials initially appeared divided on the practicality of this plan. On one hand, they said, negotiations on such a guarantee could drag for months and founder because of financial pressure on state and local governments. On the other hand, they said, a short-term fix may be possible.

Fairfax County Supervisor Joseph Alexander, a longtime member of Metro's board of directors, expressed hope yesterday that the states and the city might rescue Metro temporarily by putting up a "minimal amount," perhaps $50 million, in the form of a "one-time advance" that would be repaid later.

Under this proposal, Metro would have to devise a new plan, midway between the 76.4-mile and 89.5-mile systems, that would cost only slightly more than the amount now available for Metro construction. The excess cost eventually would be repaid either from future congressional aid or from county and city funds.

Prospects of any short-term agreement are far from assured. Negotiations may stall until it becomes clear whether Congress will overturn the administration's proposal and allocate more funds for Metro. If so, delays in Metro's current plans for opening rail extensions are possible.