Ronald Reagan added to his long list of potentially expensive promises today, pledging to increase Social Security benefits for Americans over 65 who choose to continue working.

As an incumbent seeking reelection, President Carter has been lavishing federal funds on politically strategic states. As the challenger, Reagan has no federal cornucopia at his disposal, but he promises as challenger Carter did four years ago.

This week, Reagan has promised at various times in various cities to aid the steel industry, raise military pay, give tuition tax credits to parents of private school children, aid the Northeast's lagging economy, build more Navy ships, help the merchant marine, boost spending on the Voice of America, phase out inheritance taxes, and -- repeatedly -- to raise defense spending.

Many of this week's promises have been central to Reagan's campaign since its beginning, but the Republican candidate appears to have modified his views on such potentially costly issues as federal aid to New York City, bailouts like Chrysler's, occupational health policy and labor policy.

On most occasions, Reagan doesn't mention what his pledges might cost, and his enthusiastic audiences ask for no numbers.

Today, however, Reagan campaign aides put the annual cost of his latest Social Security proposal -- that benefits not be reduced because of earnings -- at $444 million.

In the Tampa Bay-St. Petersburg area, heavily populated by senior citizens, Reagan's proposal was welcomed warmly.

The Republican candidate began by pointing out that Carter accuses him of not supporting Social Security.

"That's just not true," Reagan said. "I will preserve and strengthen this fundamental contract between the American people and their government."

His proposal would do away with the existing limitation under which anyone over 65 forfeits $1 in benefits for every $2 earned over $5,000 a year.

By Reagan's calculation, the cost of additional benefits would be $2.1 billion, but all except $444 million would be offset by additional income tax revenue. His calculations do not include the impact on the job market if more elderly people continue to work.

Carter, who also was campaigning in Florida, said that Reagan's proposal would mean higher Social Security taxes "for everyone."

White House domestic policy adviser Stuart Eizenstat said that about 1.5 million people who otherwise would be eligible for Social Security benefits have incomes above the $5,000-a-year cutoff point. He estimated that removing the earnings test would cost the Social Security trust fund between $6 billion and $7 billion a year in additional payments.

Reagan, who is still a crusader for reduction of federal spending, has a plan to reduce spending by 7 percent over five years without touching anything but "fat."

"There is so much fat in the federal budget that if it were rendered into soup you could wash the world," Reagan says.

In none of his speeches this week did he indicate any government program that might be eliminated or substantially reduced, but his final speech before flying to California for the weekend gave his campaign week a certain symmetry.

After promising ships to the National Maritime Union, more broadcasts behind the Iron Curtain to Polish Americans, jobs to steelworkers, pay raises to the military, tuition tax credits to Italian- and Polish-American audiences who make heavy use of Catholic schools, Reagan's last speech was titled: "Controlling Government Spending."

At a well-attended rally on the statehouse steps in Columbia, S.C., Reagan accused Carter of failing to control waste, fraud and abuse. The GOP candidate called this "one of the most important ways to control federal spending."

"In a time of rampant inflation, this abuse of the taxpayer threatens to undetermine the faith of the American people in our domestic institutions," Reagan said.

While Reagan's list of campaign promises grows, he attacks Carter daily for "a string of broken promises and of trust betrayed."

Reagan has moved smoothly recently into unfamiliar terrain, winning support from some labor unions and aiming his campaign at other traditionally Democratic constituencies.

He embraces youth and the elderly; the poor and the rich; the worker and the factory owner.

Speaking in Bucks County, Pa., he said: "Keep in mind, this dream is not so much for those who are already well-off as it is for those -- the great majority of Americans who are less fortunate -- for the poor, for minority Americans and for the elderly. They have the most to gain when we establish economic policies which preserve the value of the dollars they earn. A truly compassionate government must be fully commited to this objective."