WASHINGTON — The Trump administration gave Congress official notice on Thursday that it plans to renegotiate Nafta but provided only the vaguest of hints about modest changes President Trump would seek to an agreement that he has called “the worst trade deal ever.”

In a brief letter to lawmakers, Robert Lighthizer, the newly confirmed United States trade representative, said the administration aimed to support economic growth and better-paying jobs through unspecified improvements to Nafta that would modernize the 23-year-old agreement. But the notice — a drastically scaled-back version of a draft the administration circulated this year — promised no major modifications of the sort that the president has hinted he will seek.

Mr. Trump had threatened to withdraw completely from the agreement, only to relent in late April when the leaders of Canada and Mexico, the other parties to the deal, called and asked him to renegotiate instead. Sonny Perdue, his secretary of agriculture, also presented Mr. Trump with a map illustrating the potential negative consequences for American farmers if the deal were shut down.