The best audience response to last night's second Congressional Leadership Debate came at the end, when the four participants had finished their predictable speeches about the economy and started trading barbs.

"We all want to help Tip O'Neill out," said Sen. Robert J. Dole (R-Kan.), referring to the Democratic Speaker of the House.

"And one way to help him out is to send more Republicans to Congress."

When the laughter in the House Caucus Room subsided, Dole went on to declare that President Reagan has been "fair and flexible" in his economic proposals.

"I can't believe you said that," responded Sen. Dale Bumpers (D-Ark.). There was more laughter, and moderator Marlene Sanders of CBS News brought the debate to a close.

She had summed up the evening right after the first exchange of the hour, in which the legislators tried to tackle unemployment but failed. "I really didn't take much comfort from this discussion so far," she said.

The League of Women Voters tried to let its nationwide television audience hear members of Congress "articulate their party's positions and philosophies on major national issues" as part of the election campaign. But the debaters exchanged so many compliments that it was sometimes hard to tell the sides without a scorecard.

Dole praised Rep. Thomas S. Foley (D-Wash.), the House majority whip, for backing the administration's job credit program, and Foley praised Dole, chairman of the Senate Finance Committee, for backing more unemployment benefits. Bumpers commended Dole for speaking out on the problems of Social Security financing, and everybody agreed when Rep. Barber B. Conable Jr. (R-N.Y.) asserted that the government is "out of control."

The league gave each side 2 1/2 minutes to speak on each of four issues: unemployment, taxes, Social Security reform and the role of government in American life. Bumpers, summing up, said the nation "is on the brink of economic catastrophe," but Dole said it is "on the threshold of recovery." The two sides agreed that the current situation is not the best.

The sharpest exchange came over Conable's assertion that "nobody is advocating cutting Social Security benefits. People must understand that."

Foley disagreed. The Reagan administration, he recalled, proposed a $40 billion reduction in benefits last year. "A certain amount of anxiety" lingers from that unsuccessful effort, he said.

Dole was unperturbed. "Maybe we ought to get Walter Cronkite to make the announcement" that there will be no cuts, he said. "People will believe Walter Cronkite."

Bumpers came as close as any of the speakers to a concrete proposal on any of the topics when he said Democrats want to postpone the tax cut due next year and to set up a jobs program that will put 500,000 people to work. He wanted to elaborate but he had used most of his time summing up the problem.