WASHINGTON — Hillary Rodham Clinton said she always expected tough competition in the Democratic presidential primary. It’s likely she 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 today in Las Vegas for the party’s first debate of the 2016 campaign. The senator and the former secretary of state will be joined by a trio of candidates who occupy the basement of early polls, each looking to change their fortunes with a breakout moment in prime time.

For months, Clinton and Sanders have circled each other cautiously and avoided personal attacks. But in recent days, both have shown that their preference to focus on policy doesn’t mean they won’t find ways to jab at each other.

Sanders, who has filled arenas with crowds in the thousands and matched Clinton’s fundraising take in the past three months, has cast the former secretary of state as a late-comer to the liberal positions he’s held for decades on education, the environment and the economy.

After Clinton announced her opposition to a sweeping Pacific Rim trade deal, a pact she had previously called the “gold standard,” Sanders said he was glad she’d come to that conclusion. Then he added: “This is a conclusion I reached on day one.”