The predecessor, Mr. De Gucht, had sometimes come across as “arrogant,” Ms. Stoczkiewicz said. “So Malmstrom might have been brought in to present a kinder face but still to push forward” the trade agreement.

Linas Linkevicius, the foreign minister of Lithuania, said on Wednesday that, while he did not know Ms. Malmstrom’s views on trade, he would assume that because Sweden is a strong supporter of the pact, she would be, too.

“It’s a question what she will do,” he said during an interview in Vilnius, but added that when he had met her to discuss other issues, “she was very rational, very smart.”

The problems she faced as home affairs commissioner might seem easy in comparison with her new mission, in which convincing a skeptical European public that a United States deal is in their interest will be a difficult task.

The Transatlantic Trade and Investment Partnership, as the United States-European Union proposal is called, moves beyond the approach of free-trade deals of a generation ago that were concerned mainly with reducing tariffs. It aims for no less than eliminating barriers like the two regions’ differing standards on chemical products, drugs, auto safety and food — all areas in which constituencies on both sides of the Atlantic have substantial vested interests — to create a vast zone in which goods and services flow unimpeded. It also includes a highly contentious proposal to allow corporations to sue governments for lost profits when they believe that local regulations have impaired their business.

It is no secret that many citizens lack confidence in the European Union to negotiate any such deal, and there has been wide criticism of a lack of transparency in the process. Europeans worry that corporations will use an agreement to water down labor and environmental standards, and to open the doors to imports of American hormone-treated beef, genetically modified foods and other such affronts to European sensibilities. Ms. Malmstrom will need to convince the public and some of the less-enthusiastic member states that she can negotiate a deal that takes their concerns into account.

Ms. Malmstrom said in a statement: “Trade is a vital part of Europe’s economic recovery, and a cornerstone of our prosperity.”