Just like Donald Trump was the lightning rod in the first Republican presidential debates, Hillary Clinton finds herself at center stage for the lead-off Democratic debate Tuesday night in Las Vegas -- where the showdown likely will revolve in large part around her. 

While the Democratic front-runner spent the first months of her campaign dodging controversies ranging from Benghazi to her personal email use, she's also come under fire from primary rivals who suggest she's testing the political winds before taking positions on hot-button issues. 

Tuesday night will be their first chance to directly confront her about it. 

"I didn't come out opposed to the Trans-Pacific Partnership just on the eve of a debate and didn't do it because polling told me to do it. I came out against it eight months ago because I thought it was not in the best interest of the country," former Maryland Gov. Martin O'Malley said recently at a stop in Iowa. 

The 2016 White House candidate was referring to the Pacific nation trade deal that Clinton once advocated for as secretary of state but now opposes. Her position marked perhaps her biggest break with the Obama administration, and fueled criticism from Democratic rivals that she's arriving late to positions they've held for years. 

It's unlikely the CNN debate will see the kind of fireworks that Trump brought in his sparring with rival Republican candidates. But aside from O'Malley, Vermont Sen. Bernie Sanders also has shown a willingness to take on the front-runner. 

While Clinton and Sanders have circled each other cautiously and avoided personal attacks for months, both have indicated 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 the 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." 

"I think they have been a disaster for the American worker, a lot of corporations shut down here and move abroad," Sanders said of trade deals like NAFTA, on NBC's "Meet the Press." "So people will have to contrast my consistency and my willingness to stand up to Wall Street and corporations with the secretary." 

Clinton's stance on TPP was announced just days after she came out against the Canada-to-Texas Keystone XL pipeline. While she did not formally take a position on that project as secretary of state, she did appear to indicate reluctant support while in that role. In 2010, then-Secretary Clinton told a San Francisco audience, "We're either going to be dependent on dirty oil from the [Persian] Gulf or dirty oil from Canada." 

Clinton also has changed position to support same-sex marriage. As a New York senator, she opposed it, saying in 2004 that marriage is a "sacred bond between a man and a woman." And she has described her Senate vote authorizing use of force in Iraq as a mistake. 

Clinton allies suggest the switches won't hurt her. Brad Woodhouse, president of the pro-Clinton Correct the Record, told Fox News that voters prefer a leader whose views "evolve" over GOP candidates "living in the 1950s." 

"I don't think this is a problem for her," he said, adding that she's moving "closer to where the American people are." 

But Matt Schlapp, chairman of the American Conservative Union, said Clinton's policy changes are more about "cozying up" to liberal figures like Sanders and Massachusetts Sen. Elizabeth Warren. 

"Every major issue, she has shifted," he said, claiming that's why those in the base "kind of like her, but they do not love her." 

Five candidates will face off Tuesday night. Aside from Sanders and O'Malley, the stage will include Jim Webb, a former Virginia senator, and Lincoln Chaffee, a former Rhode Island governor and senator. 

Not expected to be on stage is Vice President Biden, who continues to mull a presidential bid. 

According to a White House official, Biden plans to host a high school reunion on Tuesday and later watch the debate at the Naval Observatory residence. 

Clinton's performance could weigh on Biden's decision. 

While Clinton remains the front-runner in the race, a new Fox News poll underscored her potential vulnerability in a general election -- and showed Biden faring better than her against would-be GOP rivals. 

In hypothetical 2016 matchups, Clinton trailed all the Republicans tested, including Ben Carson by 11 points and Trump by 5 points. By comparison, Biden was leading Trump by 13 points and Carson by 4 points. 

The poll was based on interviews with 1,004 randomly chosen registered voters from Oct. 10-12. The poll has a margin of error of plus or minus 4 points for the head-to-head match-ups. 

The Associated Press contributed to this report.