Bernie Sanders (AP/Michael Dwyer)

The surprise rivalry between front-runner Hillary Rodham Clinton and insurgent Bernie Sanders will be at the forefront as Democrats take the stage Tuesday in Las Vegas for the party's first debate of the 2016 presidential campaign.

Sanders, a self-described democratic socialist who is calling for a "political revolution," has emerged as the toughest competition for Clinton, though she remains the overwhelming favorite to win the Democratic nomination.

The Vermont 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.

Hanging over Tuesday's contest will be the shadow of Vice President Joe Biden, who is flirting with a late entry into the Democratic field and is expected to announce his decision within days.

Despite the Biden speculation, the Democratic primary has lacked the drama of the Republican contest and the unexpected rise of Donald Trump, the real estate mogul and reality TV star who is topping the polls but remains a longshot for the nomination that is still months away.

Clinton holds a double-digit lead over Sanders in national and many state polls. But Sanders has pulled close to her in polls in Iowa and leads in New Hampshire, two key states whose contests will kick off the primary process early next year.

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."

Indeed, Clinton has increasingly moved to the left on domestic policy since announcing her campaign this spring, including voicing opposition to the Keystone XL oil pipeline from Canadian oil fields to Gulf Coast refineries and support for expansive gun control legislation. While she rarely mentions Sanders by name, she's suggested her proposals are more realistic and well-formed than those espoused by the Vermont senator.

For Clinton, a policy-heavy debate would be a welcome reprieve from the months of focus on her use of personal emails and a private server during her four years as secretary of state. The controversy has overshadowed virtually every other aspect of her campaign and contributed to a decline in her favorability rating with voters who increasingly view her as untrustworthy.

Maria Cardona, a Democratic strategist who worked for Clinton's failed 2008 White House campaign, said that as long as the email issue doesn't dominate the debate, "this will be a win for her no matter how you look at it."

Also on the stage will be Martin O'Malley, the former Maryland governor who sought to be Clinton's chief rival. O'Malley has been sharply critical of what he sees as Clinton's flip-flopping on policy and has also said questions about her email use are legitimate.

Also looking for a breakthrough will be former Virginia Sen. Jim Webb, a decorated Vietnam veteran who criticized Clinton for her early support of the Iraq war. Rounding out the five-person field is former governor and senator Lincoln Chafee, the Republican-turned independent-turned-Democrat from Rhode Island.

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