The Reagan administration has relented under strong political pressure from proponents of the synthetic fuels program and has assured sponsors of a giant coal gasification project that they will get a $1.8 billion loan guarantee by mid-March.

Energy Secretary James B. Edwards, who last week indicated that the project -- the Great Plains plant, to be built in Beulah, N.D. -- would be scrapped, said yesterday that the venture will receive financial backing from his department after all.

Some members of Congress see the administration's shift as a sign that Reagan may be backing away from his opposition to federal subsidization of commercial-scale synfuels projects.

But administration officials insist Reagan intends to stand by his plan to eliminate Energy Department subsidies of such projects and to revise the role of the fledgling Synthetic Fuels Corp.

Although the Great Plains project emerged unscathed from administration budget-cutting, its sponsoring consortium of pipeline companies faces another hurdle -- resolving a lingering legal dispute over the price the plant's customers will have to pay for the high-quality synthetic gas.

The consortium was unable to seal an agreement at a meeting yesterday with negotiators from General Motors Corp., the nation's largest gas user, and with state officials from Ohio, New York and Michigan.