D.C. Del. Walter E. Fauntroy, who twice has backed candidates for mayor who have lost, endorsed Mayor Marion Barry for reelection yesterday.

Fauntroy, who favored former council chairman Sterling Tucker over Barry in the 1978 Democratic primary, chuckled and smiled when he told about 150 of Barry's campaign workers and supporters that he felt the need to "make public the difficult choice I . . . must make in September in voting for one among several desirable choices for mayor."

Later in the day, the other Democratic candidates for mayor were in Southeast Washington for a "Poor People's Candidates Forum," sponsored by Voice of the Ex-Offender, a group headed by local television and radio personality Ralph (Petey) Green.

All the candidates but Barry attended the forum and one of them, Ward 4 Council member Charlene Drew Jarvis, reminded the audience of more than 100 at Ballou High School of the incumbent's absence.

"I hope you notice who isn't here to hear what you have to say about what is important to you," said Jarvis. "I'll never take any area of the city for granted," she said. "When I'm mayor, I'll come back to see you."

Fauntroy's endorsement of Barry was announced during a speech to Barry supporters at the Faith Congregational Church on South Dakota Avenue NE.

Fauntory said he admires both Barry and Barry's leading challenger for the Democratic nomination, lawyer Patricia Roberts Harris.

But Fauntroy said he believes Barry should be able to finsih the work he has started as mayor.

"It would be better for me to stay out of this whole thing," Fauntroy said after the speech. "When you endorse someone, you alienate some of your own supporters. But I believe in this difficult time for the city and the country that experience in running the city government is important."