The outcome of this key procedural vote had been in doubt as a group of 14 pro-trade Democrats weighed whether to continue their support of the bill out of concern that a related workers’ assistance package might not pass both chambers.

But after repeated assurances by GOP congressional leaders that workers’ assistance measure will be adopted, 13 out of 14 backed the bill.

The vote was 60 to 37, passing by the slimmest margin needed to pass.

A final Senate vote on fast-track could come as soon as later Tuesday, and it will then head to President Barack Obama’s desk for his signature.