President Reagan made a striking retreat this week from his 1984 campaign promise never to cut Social Security benefits, stepping back into one of the most volatile political issues of his career.

He also gave up his rigid opposition to any defense spending slowdown, accepting a slower growth rate for the Pentagon than he had insisted earlier was absolutely necessary. The shifts were made in a budget compromise with Senate Republicans in which Reagan agreed to cut Social Security cost-of-living increases roughly in half under current economic projections.

Surprisingly, impetus for the limit on benefits came not from Congress but from the White House. Asked who proposed it in the budget negotiations, chief of staff Donald T. Regan said, "I did, on behalf of the president."

Then-Democratic presidential nominee Walter F. Mondale charged last fall that Reagan had a "secret plan" to cut Social Security benefits, and Reagan responded that he would "never" reduce benefits for those now receiving them.

In the Oct. 7 Louisville debate with Mondale, Reagan said:

"With regard to Social Security . . . I will say this: A president should never say 'never.' But I'm going to violate that rule and say 'never.' I will never stand for a reduction of the Social Security benefits to the people that are now getting them."

White House spokesman Larry Speakes said Oct. 10:

"The president will never stand for a reduction of Social Security benefits for anybody." Speakes said Social Security cost-of-living adjustments (COLAs) were covered by the president's pledge.

In a Nov. 3 campaign appearance in Milwaukee, Reagan said:

"I want to make one thing plain . . . . There is no secret plan to do anything about depriving people who are dependent on Social Security, and there never will be as far as I have anything to say about it."

After the November election, Reagan began to soften his stand. In a Jan. 9 news conference, he said he would have to "look at" Social Security changes if faced with an "overwhelming bipartisan majority in both houses" of Congress.

The agreement this week, however, was made only with a group of senators of his own party; officials said it was necessary to bind together a package of other deficit reductions.

As he left the White House this morning for a 10-day California vacation, Reagan displayed a renewed sensitivity to the likely political fallout from his compromise.

Asked by reporters if Democrats "are going to beat up on you" on Social Security as a result of the Senate agreement, Reagan replied, "Well, if they do, they'll be lying in their teeth, as they did in 1982."

Democrats gained 26 House seats that year in a campaign in which they exploited Reagan proposals to cut Social Security benefits sharply. A compromise financing plan was later approved by Congress and Reagan, but the inflation adjustments have continued to be targets of budget-cutting.

Before this week's compromise, the Senate Budget Committee voted to freeze cost-of-living adjustments; Regan said the president would "never touch" that.

Social Security payroll taxes can be spent only on benefits.

But in calculating the federal deficit, the taxes count as revenues and the benefits as spending. Thus, a limit on benefits directly affects the size of the deficit.

The proposal agreed to this week would shave $20.7 billion from the deficit over three years.

Reagan has struggled with Social Security as a political issue for two decades, ever since he first suggested that contributions should be made voluntary in a 1964 speech.