President Barack Obama’s trade agenda is running into potentially fatal opposition within his own party on the eve of a critical vote, and there might be only one person who can save it: Obama himself. Speaker John Boehner’s plan to bring a package of trade bills to the House floor Friday is proving to be a big gamble, as both senior Republicans and Democrats are privately wondering whether they have the votes to pass several key portions. The most pressing concern is over Trade Adjustment Assistance, a program to retrain workers who lose their jobs to free trade. The initiative...