Two men, huddling on the back porch, joined together for an impromptu duet, their voices clear and carrying over the lawn as dusk gathered. Just another country tradition--except it was Charley Pride and Bill Monroe doing the harmonizing, and the back porch happened to be attached to the White House.

Together they sang an old spiritual, "A Mansion for Me," while Pride snapped his fingers, tapped his feet and hammed it up at the finale. "This is American!" he said with a grin to the appreciative laughter of a small audience.

Pride and Monroe were among about 450 country entertainers and industry people invited by the Reagans for a reception celebrating the 25th anniversary of the Country Music Association. A gala concert is set for tonight at Constitution Hall.

"You know, this is just a country music 'Who's Who,' " said three-time Entertainer of the Year Barbara Mandrell, as she marveled at the crowd, which included Loretta Lynn, Tennessee Ernie Ford, Glen Campbell, members of the Oak Ridge Boys and John "Dukes of Hazzard" Schneider.

"I'm looking for Roy Acuff," said Vice President Bush, standing on tiptoe and craning his neck to search above the crowd. "I'm a fan of his; he's an old friend, too."

The evening marked an anniversary and as such stirred some old memories.

"I go back with your type of music to before there was an association," Reagan, wearing a plain brown suit, told the crowd. "It was WHO, a clear-channel station in Des Moines, Iowa. As a matter of fact, we had a program director who wasn't sympathetic to that music . . .

"Your music belongs here and you must admit it sounds pretty good bouncing off all this history," Reagan said, adding that in country music one could "hear the beauty of the wide open spaces, people with hearts as full as the country they live in."

Sam Marmaduke, chairman of the Country Music Association, brought gifts for the Reagans, including two cases with autographed record albums by each performer participating in tonight's show and a pair of black-and-white satin tour jackets embroidered with the Reagans' names. "I'm told that those will get you backstage at just about any concert you want to go to," Marmaduke joked to the president.

"I told the president that I would love to read Robert Service's poetry to him," said Minnie Pearl, a 42-year veteran of the Grand Ole Opry, in reference to Reagan's favorite poet. Pearl was dolled up in a cream suit, but minus her trademark hat. "He said, 'I'd love to'--which, of course, he won't remember," she quipped.

Hugging and kissing in the front hall was a trio of country music's legendary women: Tammy Wynette, Loretta Lynn and Brenda Lee.

"I'm gonna go back to the hotel and change back into my jogging suit," sighed Wynette, who wore a full lavender satin skirt, white lace and pearl blouse and beaded belt. "I'm just not much of a partier, have to rest up for the concert tomorrow. Being at the White House is always a knee-knocker, too." Wynette had just come from chatting with Agriculture Secretary John Block, who last month serenaded guests at his birthday party with a medley of his favorite country songs.

Country-rock fiddler Charlie Daniels, CMA co-chairman with Mandrell, spent part of yesterday lobbying on the Hill in support of legislation to combat record rentals and to get royalties imposed on blank tape sales. Daniels, stroking his graying beard under a gray suede not-quite-ten-gallon hat, said, "I thought it went right well."