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

U.S. Sen. Bernie Sanders defines his opinion during a press conference following a speech at the 59th annual Nevada State AFL-CIO Constitutional Convention at the Luxor Hotel & Casino on Tuesday, Aug. 18, 2015, in Las Vegas. (LE Baskow /Las Vegas Sun via AP) LAS VEGAS REVIEW-JOURNAL OUT; MANDATORY CREDIT (The Associated Press)

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Democratic presidential candidate Sen. Bernie Sanders, I-Vt., gestures during a speech Tuesday, Aug. 18, 2015, on the University of Nevada campus in Reno, Nev. (AP Photo/Scott Sonner) (The Associated Press)

U.S. Sen. Bernie Sanders defines his opinion during a press conference following a speech at the 59th annual Nevada State AFL-CIO Constitutional Convention at the Luxor Hotel & Casino on Tuesday, Aug. 18, 2015, in Las Vegas. (LE Baskow/Las Vegas Sun via AP) LAS VEGAS REVIEW-JOURNAL OUT; MANDATORY CREDIT (The Associated Press)

LAS VEGAS – 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.