Joan Mondale, the administration's arts advocate, was alone at the witness table, pleading for more support for the arts. She cited the dramatic rise in theatre and museum audiences, then quoted Carl Sandburg (always a good ending for a worthy cause) and added: "With your help, the arts and humanities can ensure that we have a chance to broaden our lives, to dance, to sing, to tell a story."

A loud "good" was heard in the room. And, two rows back, Agnes DeMille, the grande dame of chereography, who had shouted, beamed her approval.

Yesterday's Senate hearings on the reauthorization of the National Endowment for the Arts, part of the federal arts complex, went like that. The witnesses, Mondale, DeMille, actor James Earl Jones, opera singer Jerome Hines and pollster Louis Harris, were all in harmony on the need for more financial assistance for the arts.

Their potential protagonists, the subcommittee on Education, Arts and the Humanities of the Senate Committee on Labor and Human Resources offered little resistance to their positive testimony. Sens. Caliborne pell (D.R.I.), Roberts T. Stafford (R-VT.) and Jacob Javits (R-N.Y.) attended the opening day of three days of hearings. The House hearings on appropriations for the National Endowments for the Arts and Humanities were held in April and May.

In the morning session, the non Washington witnesses seemed amused at the bell-ringing system for Senate votes.At one point, Hines was explaining with emotional theatrics about the scarcity of jobs for opera singers in the United States. "We have more people earning livings in West Germany. Not more than 200 opera singers earn full-time livings here," he said, as the bell rang and Pell got up to leave. "Please keep your head of steam up," Pell said. Hines got up, faced the crowded room and took a bow.

Throughout the testimony, the witnesses hammered away about the growing constituency for the arts.

"Dance is today having the greatest unprecedented success in all of history," DeMille said, "In the early '60s, there were about a million in the dance audience and these situated mostly in New York City . . .In the late '70s, there is an audience of an estimated 20 million, scattered everywhere, and the increase is largely due to television.

"If we want the best, you have to give the best. The United States pays $1.10 for every citizen for art annually. England pays $4. France pays $10. Little Denmark pays $20 . . .And there is no possible way of estimating how much Russia spends," DeMille continiued. "Suppose (Pope) Juilius II had said to his painter, 'Hey Mike, that's enough, get a matching fund, a challlenge grant.' Then what would have happened?"

Livingston L. Biddle Jr., the chairman of the Arts Endowment, who is requesting a $5 million increased in his budget to $154.4 millions, also recited several statistics on audience interest and growth in the arts. "now what percentage of these people go by automobile?" Pell asked. Biddle looked confused for a moment, then answered: "Well I think it's a dwindling number but I think most heaters are making an effort to incorporate public transportation."

In a report last month by the House Appropriations Commitee, the Endowment was accused of mismanagement, and decisionmaking by a "closed circle" of advisers. Biddle replied: "We have been strengthening our panels. This year 56 percent of the people who serve as advisers, who serve on panels, are new members. I would hardly called it a closed circle."

In two other areas - arts education and the equity of states grants - Biddle said the agency was changing its procedures. He will soon announce the creation of an ombudsman for arts education issues between the Office of Education and the Endowment, Biddle said. The new partnership within the chairman's office will be link between the states and federal arts agencies, and will assure fairness in the discretionary grants to the States, which are increasing from 2 to 50 percent, heexplained. CAPTION: Picture, Agnes DeMille and James Earl Jones; by James K.W. Atherton