The annual convention of the nation's largest black church, the 7-million-member National Baptist Convention, USA Inc., got under way here yesterday morning with political appeals to vote -- and vote Democratic -- in the November election.

"I want a new president in the White House," Mayor Marion Barry told the delegates in his welcoming remarks. He went on to assure them that "There is nothing wrong with mixing this convention with politics, because the black church has been our political arm."

Barry, D.C. Del. Walter E. Fauntroy, who is also a Baptist clergyman, and D.C. City Council Chairman David Clarke all larded their words of welcome to the Baptists with harsh criticisms of the Reagan administration and reminders to vote on Nov. 6.

Later, in a press conference, the NBC president, the Rev. Dr. T.J. Jemison, observed that "the black church has always been in politics." Jemison indicated support for the Democratic ticket even though he said, "We will not endorse a candidate."

Democratic presidential candidate Walter Mondale is scheduled to address the convention's 30,000 delegates at the Convention Center this morning.

When Jemison announced yesterday that President Reagan had also been invited to address the convention, there were murmurings and what Jemison called "a few little remarks" of disapproval from some delegates. "But he is president of the United States, and whether we agree with all his policies or not is not important," Jemison said in defense of the invitation.

A White House spokesman said yesterday afternoon, however, that Reagan is not scheduled to address the convention of the nation's third largest Protestant denomination.

Jemison, who was one of the most ardent supporters of Jesse Jackson -- "He is one of ours" -- during the primaries, was less than passionate in yesterday's press conference in his support for Mondale. "I'm not altogether pleased with the Democratic platform," he said. He cited the Democrats' failure to oppose abortion and added, "I don't believe they gave everything to black people that they should have."

But when a questioner suggested that the disillusionment of blacks with the Democratic platform could contribute to a Republican victory in November, he shot back, "I'm not that disillusioned."

Jemison said his church had spent "approximately $800,000 to help Rev. Jackson" through voter registration efforts. "We've already registered more than 2 million. Between now and November, we'll register over a million more," he said.

Earlier this week, the Rev. Dr. Joseph Jackson, who Jemison unseated two years ago as head of the NBC, formally endorsed President Reagan, as he had also done four years ago. Jackson, a staunch conservative whose opposition to the civil rights struggle and the Rev. Dr. Martin Luther King, Jr., split the church in 1961, had endorsed a number of Republican presidential candidates during the 29 years he headed the church.

Jemison said yesterday that nine of the congregations that left the denomination in 1961 have returned during the past year, and 11 previously unaffiliated Baptist congregations -- including Fauntroy's New Bethel Baptist -- have also joined.

In his greetings yesterday, Mayor Barry paid glowing tribute to black churches. He said "I owe all of my character and all of my virtues and all of my whatever I have to the black church, because my mother made me go to church . . . I have been believing ever since."

Asked whether he supported school prayer, Jemison called it a "buggaboo question" and said "I think we need to argue about prayer in the home . . . I don't send my child to school to learn to pray."