Democratic presidential nominee Walter F. Mondale, trailing by 13 points in a new Maryland poll, brings his campaign to the state today for the first time since last May's primary.

Mondale is to appear at the Rockville Jewish Community Center this morning for what is billed as an address on issues of concern to the elderly. He will be accompanied by Rep. Claude Pepper, the 84-year-old Florida congressman who has become an aggressive spokesman for the nation's elderly.

The former vice president's visit coincides with the release of another poll showing him trailing President Reagan in a state that once was viewed as solidly in his column.

Conducted by the University of Maryland Survey Research Center, the poll has Reagan leading Mondale 49 to 36 among Maryland voters, with 15 percent undecided. The telephone survey, which had a margin of error of plus or minus 5 percentage points, was conducted between Sept. 22 and Oct. 3 among 402 registered voters who said they are likely to cast ballots on Nov. 6.

The poll, weighted to reflect the state's population characteristics, also showed Reagan with a substantial lead over Mondale among male voters, who prefer the president 56 to 30. The two candidates were tied among women and voters with union members in their households. Mondale was preferred by 80 percent of black voters, and Reagan by 6 percent.

Though Maryland is a heavily Democratic state carried by President Carter in 1980, Mondale has yet to make any headway in the public opinion surveys. His rating has slipped since a Baltimore Sun poll, conducted between Sept. 4 and Sept. 8, showed him trailing by nine points. In a University of Maryland Survey Research poll taken in July, Reagan led by only six points.

Leaders of the Mondale campaign in Maryland remain upbeat about their candidate's chances here, arguing that the state's voters traditionally turn their attentions to national politics only in the last few weeks of a campaign.

"We think we can turn it around," said Mike Frazier, state coordinator for the national Democratic ticket. "We're still very confident we can win in Maryland."

Frazier said that the combination of Sunday's presidential debate, and appearances by both Mondale and vice-presidential candidate Geraldine A. Ferraro would give the state campaign a boost.