Hillary Clinton was a full-throated supporter of the big Trans-Pacific Partnership trade deal, just sealed by President Barack Obama, when she was his secretary of state. A trade pact with Japan and 10 other countries bordering the Pacific would be a big geopolitical plus for all involved, as she saw it. Now a presidential candidate in a Democratic primary race heavy with TPP critics, Mrs. Clinton has become the trade agreement’s best-known opponent.

Here is a short walk through her varying views on the subject.

Sep....