FOR A CHILLED NATION, President Carter's fireside talk Wednesday night was a warm bath. Mr. Carter's informality accomplished its evident aim of confirming that the tone and manner of national leadership have changed. The new chief executive - away from an imposing desk, away from flags and presidential seals and business suits - came across as a kind of concern, good-natured uncle, earnestly sharing his thoughts with the country in a mature, down-to-earth way.

While the format derived from Franklin D. Roosevelt, the content was quintessentially Carterian, from the emphasis on "competent and compassionate" government to the request for citizens' opinions and help - but not their gifts. Moreover, it was Mr. Carter at his calmest, calling to mind the less frenzied and more appealing phases of his campaign. Indeed, the broad policy declaration in his speech was that he has not forgotten that campaign and does not regard his many promises last year as so much baggage that can be jettisoned at his convenience along the way. Instead, he has carted those commitments into the White House and installed them as goals against which his performance should be judged. It amounted to a renewed pledge of adherence to the lines that he laid down last year and in last month's inaugural address. In his low-key discussions of the proposals that have been refined - including his economic package and the need for reorganization authority - there was also hints of a firmness that is likely to be tested by Congress rather soon.

For this week, however, the President's main message was aabout turning down the heat and the political rhetoric as well. It was about public cooperation and shared sacrifices, and the need to tackle large problems that cannot be wished away or quickly solved by government. It is a message, we suspect, that is well suited not just to the fuel crisis of the moment, but also the general public mood.