As union officials convene at the Democratic National Convention in Philadelphia, the president of the largest umbrella labor group in the United States is bracing for a battle against a Republican nominee who is making a direct appeal to union members.

In a wide-ranging interview about the 2016 presidential race, Richard L. Trumka, the president of the A.F.L.-C.I.O., spoke some of his warmest words about Hillary Clinton, the Democratic Party’s presumptive nominee for president, and predicted that appeals from her Republican opponent, Donald J. Trump, would sound hollow to working-class voters.

“I think she’s running to change America, not to manage America,” Mr. Trumka said, describing what he saw as a difference between the two. Asked if that meant he considered her to be an agent of change, he replied, “I do.”

That is a case that Mrs. Clinton will seek to make herself at the convention, which will take place this week in a battleground state that the Trump campaign is seeking to put in play in the general election because of its density of working-class white voters, to whom he is directing his message.