What to expect from the Democratic presidential debate

LAS VEGAS -- After two very lively Republican debates, it is the Democrats' turn Tuesday night.

Front-runner Hillary Clinton is, for the first time, debating her closest rival -- Sen. Bernie Sanders of Vermont -- along with former Sen. Jim Webb of Virginia, and former Governors Martin O'Malley of Maryland and Lincoln Chafee of Rhode Island.

Democratic candidates set to take center stage

Notably absent, Vice President and potential candidate Joe Biden.

The face-off pits an impassioned populist -- Sanders -- against a measured master debater, Clinton.

The two worked together in the Senate for eight years. So far, their attacks on each other have been more like love taps.

Sanders' aides say he will focus on his policy differences with Clinton -- though they note, there aren't as many differences as there used to be.

Clinton has joined the surging Sanders in opposing both the Keystone XL oil pipeline and a pending trade deal with Asian nations -- something she supported as secretary of state.

"What I know about it as of today, I am not in favor of what I have learned about it," Clinton told PBS last week.

What to expect from the first Democratic debate

The other three candidates are all at one or two percent in the polls and looking for a breakout moment. And that lends an air of unpredictability to a debate that is still sure to be less volatile than the first two GOP outings.

Supporters working to draft Biden released a new ad Tuesday, trying to keep him in the conversation.

There's a side debate going on about the number of debates. Some of the candidates and some top Democrats want more of them. But the Democratic Party chair is holding firm, saying there will be just six in the run-up to the 2016 election.

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    Nancy Cordes is CBS News' congressional correspondent.