One of the first casualties of President Reagan's decision yesterday to withdraw support from Philippine President Ferdinand Marcos was the high-powered Washington lobbying firm of Black, Manafort, Stone & Kelly, which was forced to drop its $900,000 contract to represent Marcos' interests here and his struggle for political survival at home.

A spokesman for the firm, most of whose partners are former Reagan campaign aides or Republican Party strategists, read a statement yesterday to reporters who inquired whether the firm would continue to represent Marcos in light of new allegations of election fraud by former Marcos military commanders.

"At the time we undertook this representation, the Marcos government was a longstanding ally of the United States," the spokesman said. "In light of President Reagan's statement today calling for a transition to a new government in the Philippines, we can no longer effectively represent . . . [Marcos'] interest in the United States."

"Therefore," the statement concluded, "we have today resigned the . . . account effective immediately."

Though the firm's contract was signed with a chamber-of-commerce style collection of Filipino manufacturers, exporters and tourism agencies, yesterday's statement acknowledged that "The interests of the chamber are closely allied with those of President Marcos."

A number of congressional and news media representatives who were in the Philippines during the weeks before the election said that partner Paul Manafort "was everywhere" arranging interviews and meetings with Marcos and his top advisers.

As charges of election fraud mounted in the Philippines, the role of the firm has "raised a lot of eyebrows" in Congress, according to one Senate aide. Several Senate sources said yesterday that Republican candidates who have retained the political consulting services of the firm were concerned that Democratic challengers will use the Marcos connection in campaign attacks.

A staff aide to Sen. Paula Hawkins (R-Fla.), who has retained firm partner Charles Black for political advice in her race against Florida Gov. Robert Graham (D), said yesterday that Hawkins was not concerned about the connection. "Black has never discussed the Marcos situation with Paula," the aide said.