Walter F. Mondale, in his first fall campaign appearance in Maryland, promised a group of senior citizens yesterday that his administration would not take "a single dime" from Social Security and Medicare benefits now or in the future.

The Democratic challenger, speaking to several hundred enthusiastic supporters at the Jewish Community Center in Rockville, said 73-year-old President Reagan has been a "travesty" for senior citizens and he chastised Reagan for breaking his 1980 campaign promise to protect senior citizens' benefits.

"The first time they cut Social Security and Medicare after promising not to , it was their fault," Mondale said during a 20-minute extemporaneous speech. "Next time they tell you they're going to cut them and you let them, it's your fault."

Mondale's visit to Maryland came on the heels of two independent polls showing that he trails Reagan by as much as 13 points in the heavily Democratic state. Yesterday's appearance was a calculated effort, Democrats said, to get the attention of media in the Washington suburbs, which are key to a Democratic victory in Maryland in November.

Maryland was one of six states Reagan lost in 1980, and early in this general election campaign Democratic strategists said a Mondale visit was unlikely. As part of the drive to shore up ebbing support statewide, vice presidential candidate Geraldine A. Ferraro visited Baltimore last week.

Monday, Reagan is scheduled to visit Baltimore's blue-collar Litte Italy, a traditional Democratic stronghold, to dedicate a statue of Christopher Columbus.

Flanked by the state's top Democratic politicians -- including Gov. Harry Hughes, Attorney General Stephen H. Sachs, U.S. Sen. Paul S. Sarbanes, and Reps. Michael D. Barnes of Montgomery County and Barbara A. Mikulski of Baltimore -- Mondale described the Reagan administration as cruel to Americans over 65 years old.

Using a term coined by former Democratic vice president Hubert H. Humprhey, whom he described "as almost a father to me," Mondale said Reagan had failed "the moral test of government" by cutting benefits to persons "in the twilight of life . . . and in the shadow of life."

Reagan's proposed 25 percent cut in Social Security, which was blocked by Congress, a $15 billion cut in Medicare, and dropping 500,000 persons from disability rolls amounted to "official cruelty," Mondale said.

"Who were those people cut from disability rolls ?" Mondale asked. "You know these bag ladies who walk around in the street, or these pathetic people who sleep on grates. . . . These are God's people. They need just a little lifeline just to keep a little decency."

Mondale said he would "defend Social Security adjusted for inflation" for current and future beneficiaries by cutting health-care costs and by finding savings in other government programs, such as defense.

"I bet most of you in this room know where to buy a coffee pot that costs less than $7,000, which is what the Army paid for it," he said, drawing laughs from the audience.

"I thought he was very good," said Sam Bodgor of Montgomery County, who said he will vote Democratic on Nov. 6. "He brought out all of the important points, at least for senior citizens."

His wife, looking on, added: "Just quote him as saying, 'Mondale's no wimp.' "