Hillary Clinton is maintaining the momentum against the Republican presidential nominee who has fallen down the radar in the last few weeks.

Clinton clamped down Donald Trump with her malicious rejection yet of a Pacific trade pact she once lauded and an attack on Mr. Trump’s economic proposals as a crony giveaway.

Clinton made this public during the week as she held a session with Warren, Mich., and she offered little new policy but provided one of her more pointed critiques of American trade policy. She flatly stated she would oppose the Trans-Pacific Partnership, a 12-nation trade pact that is a top priority of President Barack Obama.

“I oppose it now. I’ll oppose it after the election, and I’ll oppose it as president,” said the former secretary of state.

Mr. Trump, responding during a rally in Kissimmee, Fla., on Thursday night, said her opposition to the trade pact wasn’t sincere. “If she were ever in a position to approve it, she would approve it in two minutes,” he said.