Hillary Clinton and Bernie Sanders set for first Democratic presidential debate
This combination of photos shows 2016 Democratic presidential candidates Senator Bernie Sanders and former Secretary of State Hillay Clinton (AFP Photo/)

After ignoring her chief rival for months, White House heavyweight contender Hillary Clinton steps into the ring Tuesday to confront independent Senator Bernie Sanders in their first Democratic debate of the 2016 primary cycle.

Clinton will take center stage in Las Vegas joined by Sanders and three other hopefuls, and while there is unlikely to be a dramatic clash of personalities as seen in the first two Republican debates, the spotlight is likely to be on the top two candidates.

The former secretary of state will no doubt face pressure from Sanders, who will tout the populism of his political revolution in their much-anticipated encounter.

And with the other three challengers -- former Maryland governor Martin O'Malley, ex-senator Jim Webb and former Rhode Island governor Lincoln Chafee -- struggling to gain traction in the race, they will try to generate breakout moments to show they are electable alternatives to Clinton.

Vice President Joe Biden has been mulling a run for months, but he has not announced his decision and is not expected to participate in the debate, which starts in Sin City at 5:30 pm (0030 GMT Wednesday).

Clinton is nearly eight years removed from her campaign clashes with ultimate White House winner Barack Obama, and part of her goal will be to show she still has expert debate skills even though she appears to struggle to personally connect with voters.

Along with focusing on her recently unveiled proposals on gun control, Wall Street regulations and other policies, Clinton will want to prove that she is the candidate to beat despite her numbers trending downward in recent months.

Clinton still leads nationally in polls, but she trails Sanders by nearly 10 points in New Hampshire and holds only a modest lead in Iowa, both key early-voting states.

Her lead increases in debate state Nevada, where her support has reached 50 percent compared to 34 percent for Sanders and 12 percent for Biden, according to a CNN/ORC poll released Monday.

While Sanders, a rumpled, self-declared democratic socialist, has drawn huge campaign crowds, Tuesday night will be the biggest test of his decades-long political career as he seeks to build national support.

- Contrasting with Clinton -

Clinton has rarely mentioned Sanders on the campaign trail, but they will be standing a few feet apart come Tuesday.

While Sanders said he is unlikely to level "personal attacks," he suggested he will hit Clinton on her recent position shifts, including her opposition to the Canada-to-US Keystone oil pipeline and her criticism of a huge Pacific Rim trade deal that she supported on multiple trips to Asia.

"People will have to contrast my consistency and my willingness to stand up to Wall Street and corporations with the secretary," Sanders told NBC's "Meet the Press" Sunday.

The Sanders campaign telegraphed another line of attack over the weekend by releasing a statement outlining Sanders's 2002 vote against authorizing the use of force in Iraq sought by George W. Bush, which Clinton voted for.

Obama opposed the war, and his difference with Clinton on the matter marked a defining issue in their 2008 showdown. Clinton has since acknowledged she was wrong to cast that vote.

Seeking her own opening, Clinton might use her rival's moderate position on guns -- Sanders hails from Vermont, a rural state with few firearm restrictions -- to highlight an area where liberals who seek tighter gun laws break with Sanders.

Sanders casts himself as the anti-billionaire candidate intent on reshaping an economic system he says rewards the wealthiest Americans while leaving millions mired in poverty.

Clinton will likely face questions about her use of a private email account and server while secretary of state, an issue that has dogged her for months.

She got little help from Obama shortly before the debate.

The president told CBS's "60 Minutes" program in an interview aired Sunday that while the scandal was largely "ginned up" by campaign politics, Clinton "made a mistake" using personal email and "could have handled the original decision better, and the disclosures more quickly."