Robert S. Strauss, chairman of President Carter's national reelection committee, joined District Mayor Marion Barry and other city officials yesterday in opening the D.C. Carter-Mondale headquarters in downtown Washington.

Standing beside a grinning Barry, Strauss arched his eyebrows and said, "I thought when I came here to open this headquarters that we'd have a quart of champagne -- not to break over the mayor's head, or course, but to share with him."

Joining Strauss in the cramped offices at 816 Connecticut Avenue were about 150 other Carter supporters, many of them members of a D.C. Carter-Mondale steering committee whose formation also was announced yesterday.

In 1976, Carter won the District Democratic primary with virtually no help from city politicians. Del. Walter Fauntroy and former Mayor Walter Washington each headed rival uncommitted slates of convention delegates, while Carter ran a quiet campaign focused through church groups and capped by a well-organized get-out-the-vote effort.

The president's strategy this time around for the May 6 primary has been to court local officils diligently.

Joining Struass and Barry yesterday were City Council Chairman Arrington Dixon and council members Polly Shackleton (D-Ward 3), Wilhelmina Rolark (D-Ward 8), Charlene Drew Jarvis (D-Ward 4), Willie Hardy (D-Ward 7) and William Spaulding (D-Ward 5), along with dozens of others.

"This headquarters will be a model of what a campaign headquarters should be," Strauss told the crowd. "As this office goes, so goes the nation."

Along with Carter's close friend, attorney Charles Kirbo, Strauss left early to attend to other campaign duties. He was scheduled to fly to New Orleans later for a fund-raiser, aides said.

"This office is very, very important to us," Strauss said on the way to his car. "There are those who would say the president doesn't have the kind of support in the black community he had in the past. Well, just look around."

Strauss said a Carter victory in the primary would be "a message that goes out all across America, that shows clearly the support of the black community for President Carter."