The Prince George's County NAACP Freedom Fund banquet was billed as a political soiree for politicians in this overwhelmingly Democratic county seeking votes among unassailably Democratic blacks.

Thus the 7 p.m. reception did not officially begin until the governor of Maryland, in his role as the state's highest-ranking official seeking reelection in 1982, arrived promptly at 8 last Friday.

But it was the Republican message of keynote speaker Arthur Fletcher that produced the church-style "amens" before the night was over.

The crowd seemed somewhat short of chapter president Josie Bass' official head count of 315, but she said receipts of about $20,000 made it by far the most financially successful dinner the chapter has held.

Bass said the response to the fund-raising appeal was a sign of the rising fortunes of her organization in a county where the black population in the last decade has grown from from 13 to 37 percent.

As Bass cordially welcomed Gov. Harry Hughes, Republican County Executive and newly anounced Senate hopeful Lawrence J. Hogan strode straight up the middle of the room ungreeted and quickly took his place at the end of the head table.

After prayers and introductions Hughes, who was not scheduled to speak, decided to make brief remarks. He rose to recognize several members of the state legislature, in the process overlooking Prince George's only black state senator, the only black with a bona fide political organization in the county, Sen. Tommie Broadwater.

After catching his mistake, the redfaced governor went on to deliver a standard Democratic attack on the Reagan administration.

"There are things happening in Washington that aren't very good. It's an attempt to turn back 50 years of progress overnight," Hughes said. But the magic words produced no cheers of support.

Hogan, who went to Congress in 1974 as a busing foe, was seated next to John Rosser, the NAACP chapter's expert on its controversial desegregation suit against the Prince George's school board. Like every politician there, however, Hogan said that supporting the NAACP and supporting the suit were two different things.

"I'm not in favor of it," said Hogan. "The problem is not the schools, it is the neighborhoods. We've got to concentrate on the neighborhoods."

Hogan was ready to leave for another engagement when Fletcher, a former assistant secretary of labor credited with designing the "Philadelphia Plan," which guaranteed jobs for blacks on all federally contracted construction work, began to hold forth. In a booming voice he told the rapt audience that if it were not for "my friend Larry Hogan" and other Republicans then in the House of Representatives the plan would not have succeeded, and Hogan got loud applause from the otherwise indifferent crowd.

"If we in fact are going to 'get it done in 1981' we must understand that no one party and no one group has a lock on the truth," he declared. Fletcher, a converted Reagan backer who first supported George Bush's presidential bid, dished out the message of Milton Friedman with the style of a Baptist preacher. His riveting performance prompted a standing ovation and shouts of "all right" and "yes" from the crowd.

"I'm glad he said it," said a beaming Hogan. "I think it's a great message. Jesse Jackson has said the same kind of thing for years, but he doesn't say it as well as Arthur."

Executive board member Rosser said he was not about to switch parties, but added, "I'm going to have some healthy respect for the Republicans. There is definitely room for healthy discourse among blacks on this."