WASHINGTON >> Hillary Rodham Clinton says she always expected tough competition in the Democratic presidential primary.

She likely didn’t expect it would come from Bernie Sanders, the rumpled independent senator from Vermont and a self-described democratic socialist calling for “political revolution.”

The surprise rivalry between Clinton and Sanders will be at the forefront as Democrats take the stage Tuesday in Las Vegas for the party’s first debate of the 2016 campaign.

The senator and former secretary of state, Sanders and Clinton, will be joined by a trio of low-polling candidates looking for a breakout moment in front of a prime-time audience.