Democratic presidential nominee Walter F. Mondale charged yesterday that President Reagan has a "secret plan" to reduce Social Security benefits after the election, and Reagan, expanding his past pledges, announced that he would never raise Social Security taxes or cut benefits for current or future recipients.

Soon after Mondale leveled the charge in Cincinnati, White House spokesman Larry Speakes said Reagan was broadening his oft-stated pledge not to reduce benefits for those persons now on the rolls. "The president will never stand for a reduction of Social Security benefits for anybody," Speakes said, clutching a wire-service story on Mondale's charges that he had just shown to Reagan.

Speakes acknowledged that changes may be necessary in Medicare financing in the next few years and said they would be dealt with "hopefully on a bipartisan basis with the cooperation and not the demagoguery of the Democrats."

The administration and Congress reached a major compromise in 1983 on refinancing Social Security that is expected to keep it solvent into the next decade and probably beyond without changes. But some of Reagan's Cabinet members have talked about possible benefit cuts in a second term as a means of reducing the $170 billion federal deficit.

Reagan's pledge yesterday underscored what aides described as intensifying White House fears that Mondale was scoring points on one of Reagan's most serious political vulnerabilities, continuing an offensive that the Democratic nominee launched in Sunday's nationally televised debate.

Yesterday, Mondale also returned to his theme that Reagan had unfairly hurt the poor, saying the president was "mean-spirited." At a citizens' forum in Cincinnati, he said, "You have to be careful when you elect a president. You can't just ask, 'How do they look?' You also have to ask, 'How do they feel?' It seems to me that he just doesn't understand the effects of his proposals on millions of decent Americans who have a right to expect differently."

On Sunday, Reagan promised, as he has often before, not to cut Social Security benefits for "people that are now getting them." But Mondale charged that Reagan had left open the possibility of cutting benefits to those who have not yet reached retirement age.

"What about the person who is 46 today -- can he count on those benefits?" Mondale asked.

Later, Speakes told reporters to "strike all of that" which Reagan had said on Sunday, and offered what he called an "unqualifier" to the president's two-day-old remarks. Speakes said there would be "no tampering" with the system by Reagan. Assistant press secretary Marlin Fitzwater said this ruled out both benefit cuts and Social Security tax increases beyond those already scheduled, and cuts for future as well as current recipients.

Reagan, responding at a Rose Garden ceremony to a question about whether he would protect future retirees, said, "That's exactly what I meant the other night."

Both the president and Office of Management and Budget Director David A. Stockman have this year raised the possibility of Social Security retrenchments in a second term. Reagan said in an interview March 28 that "what we need to do is a revamping of the program." Reagan said July 6 that there is a "possibility" that "many people, young people, now paying in" to Social Security "will never be able to receive as much as they're paying" in benefits when they retire.

And Stockman, in a Fortune magazine interview published last Feb. 6, said, in response to a question about cuts in entitlement programs:

"As for Social Security, you're not going to take checks out of the mail. The best you can do is erode their purchasing power by capping COLAs cost-of-living adjustments below the inflation rate. But capping COLAs will take several years to have significant fiscal effect."

Social Security has long been one of Reagan's most troublesome political issues, and the White House went into high gear yesterday to try to blunt Mondale's charge that Reagan would squeeze both the retirement program and Medicare in a second term.

Reagan held a ceremony in the Rose Garden for the signing of amendments to the Older Americans Act, including a new program of personal health education. Later, the White House announced that Reagan had signed legislation resolving a long dispute over Social Security disability.

A disability review process mandated by Congress in 1980 became a painful political issue for the administration. Current government estimates are that 490,000 recipients were taken off the rolls since 1981, and about 210,000 won reinstatement. The administration suspended the disability reviews until compromise legislation, which Reagan signed yesterday, could be approved.