Prince George's County Executive Lawrence J. Hogan formally announced his candidacy for the U.S. Senate yesterday, pledging his support for the overall Reagan economic program while emphasizing that he doesn't "buy all of it."

Hogan, a former FBI agent and three-term congressman from Prince George's, said he decided to run for the seat presently held by Democrat Paul Sarbanes despite urgings from his supporters that he run for reelection to county executive. Hogan, 53, said his travels throughout the state as a Republican national committeeman have convinced him that Sarbanes is out of touch with constituents and that his ideas are "out of date."

"He clings to the same worn-out rhetoric in support of programs that did not work in the past and would not work in the future," said Hogan, the only Republican holding elective office in Prince George's.

Hogan said he "substantially" supports President Reagan's policies and urged that Reagan's ideas be given more time to work. "I don't buy it Reagan's program completely, but I don't think there's any question that the burgeoning national deficit needs to be addressed."

He cited unemployment and inflation as two of his chief concerns and said he favored a strong national defense.

Hogan represented Prince George's in Congress from 1968 until 1974, when he lost the Republican primary in a race for governor, his only previous bid for statewide office. Campaigning on a promise to cut taxes and spending, Hogan became county executive of Prince George's in 1978 after defeating incumbent Democrat Winfield M. Kelly Jr.

Accompanied by his wife, Ilona, and his oldest son and aide, Lawrence Hogan Jr., Hogan embarked on a helicopter tour of the state, starting with a morning press conference at a hotel in Calverton in Prince George's. From there he flew to another ceremony in Baltimore, then to Salisbury, on the Eastern Shore, and to Hagerstown, in Western Maryland.

In Calverton, Hogan was applauded by a group of about 20 members of the Prince George's and Montgomery Republican central committees and some elected officials.

Hogan was introduced at the Calverton gathering by John Burcham, with whom Hogan had a bitter feud over a year ago. Burcham had refused to support Hogan's son in his race for a congressional seat made vacant last year. Hogan then refused to reappoint Burcham to the Maryland National Park and Planning Commission chairmanship. Burcham said Hogan asked him last week to perform the introduction and he agreed, he said, because, "time heals all wounds." Later in the program Hogan introduced Burcham as his likeliest successor as county executive.