MR. MONDALE was very much on the attack last night, though he was careful not to trample on the dignity of the presidency. He was able to use the latest news -- the CIA's Nicaragua manual, the developing record of the administration's Lebanon travails -- to advantage. President Reagan, however, came back hard on some of the old Carter policies and did his best to tag his challenger with "a record of weakness second to none." There was a lot of vigor and some genuine strong feeling: at one point a flushed Mr. Reagan, twice (and fairly) challenged on his knowledge of arms control detail, declared he would not respond further to Mr. Mondale's "repetitions of falsehoods."

We thought Walter Mondale put on a presidential performance. He had a theme -- the requirement for alert, knowledgeable and responsble leadership -- and he illustrated it deftly as the debate went along in respect to military programs and arms control proposals alike. His pursuit of the president on personal knowledge of and commitment to arms control was sure.

Mr. Mondale's particular purpose was to rebut the notion that he would be soft in standing up to the Russians. He was pretty skillful in putting the best face on his past opposition to so many military programs. He would apply a nuclear freeze, for instance, only when it was mutual and could be verified with assurance -- this helps get him off an unfortunate wicket, i.e., the freeze. His explanation that he voted earlier to slow down the F14 because of bugs in the plane made the necessary point that, in national defense, money isn't everything. We think he was wrong to oppose the immigration bill, but he made about the best case for his opposition that is available.

Ronald Reagan made a notable gaffe by unwittingly acknowledging the previously known but unacknowledged CIA role in Nicaragua. His explanation of the Nicaragua manual was, to be charitable, lame. His suggestion that it was not he but the military commander who put the Marines in the Beirut barracks allowed Mr. Mondale to cite Harry Truman tellingly: "The buck stops here."

Yet the president has plainly come a long way from the simplicities of his campaign of four years ago. He tried hard last night to reach out to those who have been unnerved by some of his policies and who doubt his devotion to peace and, specifically, to arms control. He was right when he said that all human rights are lost when a country goes totalitarian. He was easy and avuncular, and scored well in response to a question on his age.

Mr. Reagan did much better than he had done in the last debate. He has had some successes in foreign policy; his administration has done some things well. In certain respects we would say he even had a better case than he made. Factually, he was weak. We would give the edge to Mr. Mondale.