Democratic presidential candidate Hillary Clinton speaks to supporters in Iowa on Sunday. (Charlie Neibergall/Associated Press)

AFTER WORLD War II, the United States and its allies labored mightily to construct a global free-trade regime. They did so for many reasons, not least to avoid the trade wars that had fueled international tension and, ultimately, global conflict. In other words, trade — and the prosperity and interdependence it engenders — was central to both U.S. economic strategy and U.S. security policy.

Understanding this, and hoping to extend past achievements into the 21st century, President Obama is seeking Trade Promotion Authority from Congress so as to negotiate a consolidation of trade relationships with Pacific Rim nations, the most crucial of which is Japan, to be followed by an agreement deepening U.S. economic ties with Europe. It may be a turning point in contemporary history; if the president succeeds, the United States’ leadership could be strengthened for a generation. If he fails, U.S. influence will ebb, and, with it, American prosperity and security alike.

Of all the people who should be standing shoulder to shoulder with Mr. Obama at this moment, none could do more to help him than Hillary Clinton, who was an enthusiastic champion of these agreements throughout her tenure as secretary of state. The resistance to Mr. Obama’s trade agenda is led by Democrats in the House who fear that supporting the president will earn them the enmity of organized labor and other progressive groups that simplistically denounce the president’s trade agenda as a threat to jobs and the environment. These Democrats are joined, and validated, in this short-sighted position by Ms. Clinton’s rivals for the Democratic nomination, former governor Martin O’Malley of Maryland and Sen. Bernard Sanders (I-Vt.).

Think of the bracing effect it might have had on her fellow Democrats if Ms. Clinton had stood up against election-year political pressure and reiterated what she said as America’s top diplomat in 2012: “that we need to keep upping our game both bilaterally and with partners across the region through agreements like the Trans-Pacific Partnership, or TPP.” Imagine if she had read from her memoir, published just last year under the now ironic title “Hard Choices”: The TPP, Ms. Clinton wrote, “was also important for American workers, who would benefit from competing on a more level playing field. And it was a strategic initiative that would strengthen the position of the United States in Asia.”

Instead, Ms. Clinton broke her silence on the issue only long enough to equivocate about it, or, as she did Monday, to expand on the “legitimate” concerns of opponents of the TPP — and second-guess the president’s strategy. With 18 months to go in his second term, she now advises Mr. Obama to use the House Democrats’ obstruction of Trade Promotion Authority as “leverage” against the 11 negotiating partners; perhaps to reopen it just as the Obama administration spent a couple of years redoing the U.S.-Korea trade deal it inherited from the Bush administration.

What this politically unrealistic advice did not include, however, was a simple yes or no to the question of whether Mr. Obama should have the additional bargaining power of Trade Promotion Authority now, as he requests. And that is the question of the hour.