It's hearts versus heads for many in unions as they decide whether to back Clinton or Sanders

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.