Washington — Walter F. Mondale returned to his former work place, the US Capitol, Thursday for a rally of his party's faithful that yielded an ebullient show of unity among Democratic lawmakers.
''We ought to be getting on President Reagan's back, not on Fritz's back,'' said Sen. Edward M. Kennedy of Massachusetts after the presidential nominee and former Minnesota senator addressed Democrats in the Senate.
Sen. Patrick J. Leahy (D) of Vermont said of the session, ''It's the most supportive I've seen the (Democratic) caucus act'' toward any candidate.
Only a day earlier, Democratic House Speaker Thomas P. O'Neill Jr. had complained publicly that Mr. Mondale was not ''slugging'' hard enough against President Reagan. But after the Democratic nominee spoke to the House Democratic Caucus, the Speaker proclaimed, ''Mondale did sensational,'' and added that ''if he could only talk to people head-on'' in that same manner he would win the election in a landslide.
Mondale collected a variety of advice from the lawmakers, including a call to be more aggressive, to plan more events at factories and farms rather than at airports, and to demonstrate his accessibility to people.
The show of support comes at a low point in the Mondale presidential campaign: according to a just-released Los Angeles Times poll, he is 27 points behind. Such a victory margin could sweep away Democratic lawmakers next November.