In the photo, you can see President Jimmy Carter wearing a cardigan in the White House to demonstrate that he was practicing energy conservation. In the 70s this was depressing for many Americans who thought that energy supplies were limitless and that the USA should never be so poor that they couldn't heat the home of the President.
To clarify the vocabulary, to be clad in something means to wear it or be decorated with it, and gloom is depression. So his sweater (cardigan) was a symbol for America's depression and anxiety at that time.
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