Sen. Edward Kennedy was winding up a fiery political pep talk in the drafty cafeteria of an electronics plant, here charging Jimmy Carter with high interest rates that have killed home-buying, heating bills that threaten health and inflation that is undermining the nation.

"Do you want four more years of that?" he shouted at the middle-aged audience of electronics workers. "No," they shouted back. Insuch a setting, as in his brilliant handling of adoring street crowds that greeted him here and in several other New Hampshire towns last week, Teddy Kennedy was superlative.

But beyond the surface brilliance, a multitude of political problems, from uneven public performances to courtship of the Democratic center, besets the campaign of the third Kennedy brother. He now runs no better than even with the president, in the opinion of many Democratic politicians (a verdict more than confirmed by the public opinion polls).

Back in New Hampshire for a final 1979 campaign swing for the Feb. 26 presidential primary election Teddy Kennedy wisely ignored the crisis in Iran, which got him in such deep political trouble Dec. 2. He confirmed in a two-day swing that until Iran radically worsens -- or the crisis ends -- his campaign will be targeted

But the ease with which he moved the elderly workers at the Clarostat electronics plant here was not duplicated several hours later. Twenty miles north of here in the small town of Rochester, a Chamber of Commerce luncheon audience sat on its hands through 20 minutes of similar but less spontaneous Kennedy rhetoric on the twin crises of inflation and energy.

He charged Jimmy Carter with responsibility for the fact that "elderly people are huddled in small apartments in Rochester," unable to afford heating oil. His audience did not stir. He fumbled a line about the Kennedy energy conservation plan for "weatherizing" homes, referring to "weatherization of the program" instead of houses.

When he finished, applause was tepid and barely spontaneous. In the question period, he was asked only one question about the economy -- whether he favored tax credits for the of wood stoves (he does).

His apparent failure to move members of the Chamber of Commerce in Rochester, after arousing the unionized workers here, symbolized Kennedy's problem in winning the hearts and minds of the center and right wing of his party. That problem had brutally but privately surfaced in Washington Dec. 13, when Kennedy invited a dozen or so leading defense contractors to his McLean, Va., house for breakfast. The purpose: to try and demonstrate thatTeddy Kennedy's long record of "no" on higher defense spending did not mean he was unsympathetic to their point of view.

The breakfast, on the private testimony of several who attended, was less than successful. Kennedy himself was not there, but his choice of a representative displayed a surprising lack of knowledge about what makes defense contractors tick. Chief briefer for the senator was Mark Schneider, a former top State Department aide in promoting Jimmy Carter's somewhat controversial human rights policies.

Defense industry representatives from such well-known companies as Raytheon, Garrett, Fairchild and several others were disappointed that Schneider, in the view of one, "had little understanding of Soviet arms programs and was not at ease with the general subject of military planning and weapons building."

Adding to the disappointment of those at the breakfast was the fact that neither Stephen Smith, Kennedy's top campaign strategist, nor Sargent Shriver, like Smith a brother-in-law, was present as promised in the invitations.

Breaches like that are not unusual in any presidential campaign, particularly one as hastily put together as Teddy Kennedy's. But this particular breach cast doubt on how serious Kennedy is to show himself not as an ani-defense liberal ideologue but as a pragmatist able to cut his cloth to changing requirements.

Indeed, the choice of Schneider as one of the senator's foreign-military policy strategists, perhaps the chief defense strategist, displays a certain naivete for so polished a political performer as Kennedy. On the two-day campaign swing here, Schneider was part of the senator's traveling staff to counsel him on his foreign and military policies -- a further indication that political reality has not yet taken full possession of Kennedy's bid to run Jimmy Carter out of the White House without that "four more years."