The choice facing labor unions in the Democratic presidential race boils down to hearts or heads.

Will it be Bernie Sanders, who embraces their opposition to a big trade deal and other elements of the labor agenda? Or Hillary Rodham Clinton, who is a strong favorite for the nomination and has longstanding ties to labor?

Clinton remains the Democratic presidential front-runner. But her unwillingness to take a position on the Trans-Pacific Partnership has caused friction with unions.

Unions that endorse Clinton this year might gain more clout if she wins the White House. But some labor leaders fear a backlash from members drawn to Sanders' message and consider him a natural ally.

Clinton, Sanders and former Maryland Gov. Martin O'Malley are courting the Nevada AFL-CIO this week.