WASHINGTON — The White House and Republican leaders in Congress face long odds in trying to revive trade legislation after a telling defeat engineered by President Barack Obama’s fellow Democrats. Obama’s ambitious trade agenda is in serious doubt, as is his quest for a capstone second-term achievement. Without the power to negotiate trade deals that Congress can approve or reject, but not amend, Obama has little chance of securing the Pacific Rim pact that his administration has worked toward for years. Friday’s rebuff highlighted the strained relations between Obama and congressional Democrats, who voted down a worker assistance program crucial to the negotiating authority measure just hours after the president implored them not to. Obama and House Speaker John Boehner, R-Ohio, had worked in rare tandem on this issue, yet their inability to deliver raises the question of whether much else will get done with Republicans running Congress and Obama in the White House for the next 18 months. “This isn’t over yet,” Rep.