Pro-growth policies have been all too rare in the Obama era, so Tuesday’s Senate vote to block a filibuster against trade promotion authority is worth celebrating. The narrow margin of likely victory in both the House and Senate also offers lessons for the future of trade politics.

The victory means the U.S. can again begin to reassert trade leadership after a decade on the sidelines. The 12-nation Pacific trade talks can now get down to the hard choices, as other countries will know that Congress can’t rewrite the deal. It will have to vote yes or no. Fast-track authority lasts six years, so the next President will also have more flexibility to pursue other trade accords.

***

While the victory is bipartisan, it’s clear the bill could only pass when Republicans control Congress. The three bilateral trade pacts left over from the Bush Administration (Colombia, Panama and South Korea) were ratified only after Nancy Pelosi was deposed as House Speaker in 2010, and trade authority moved only when Harry Reid lost the Senate in 2014. The same Republicans Mr. Obama excoriates as cynical partisans put their partisanship aside to help the country.

The bad news is that the number of pro-trade Democrats is dwindling in Congress—this time to 28 in the House and 13 in the Senate with a Democratic President, compared to 25 in the House and 20 in the Senate in 2002 for George W. Bush. This reflects the ferocity of union and environmental opposition, which makes pro-trade Democrats like Rep. Ron Kind of Wisconsin and Sen. Ron Wyden of Oregon all the more laudable.

The Democratic stampede against trade also reflects the broader retreat of liberal economists and intellectuals. It’s striking how many leading voices on the left have walked away from the free-trade support they provided as recently as the 1990s.