Five things to watch in tonight’s Democratic debate

*Who’s on Stage? Will the lineup include Vice President Joe Biden? Debate sponsors were keeping a podium warm for him just in case he decides to jump into the race in time for Tuesday night’s show.

As White house staffers were firming up schedules on Monday, it looked like Biden wasn’t planning any last-minute trip to Las Vegas. There also were no signs that he’d been engaged in the kind of intense debate prep that usually precedes such a big night.

But the vice president could still be planning an attention-grabbing surprise. He could also still decide to get into the race in the weeks after the debate. Polling suggests Biden would draw many more votes from the front runner, Hillary Rodham Clinton, than from Bernie Sanders, the Vermont senator who has been her major challenger so far.

As it stands, here’s where the candidates will stand: Clinton and Sanders will have the number one and two positions on the stage, center and center right. Former Maryland Gov. Martin O’Malley, former Virginia Sen. Jim Webb and former Rhode Island Gov. Lincoln Chafee will fill out the rest of the stage.

*Tough Talk: The non-Hillary candidates have been keeping their criticism of Clinton to a minimum so far in this campaign, but it doesn’t look like that will last through the night.

Sanders says that personal attacks aren’t his style, but he has gone out of his way recently to note that he was one of the few to vote against the Iraq war. He may use the spotlight to remind Democratic voters that Clinton voted for the war – one of her major vulnerabilities in the 2008 campaign. As the emerging favorite of the party’s left-wing, Sanders has a lot to gain from such a side-by-side comparison.

O’Malley has his own incentive for going after Clinton, mainly that he needs to make a name for himself. The month of October has seen him criticizing the front runner being slow to announce her opposition to the Keystone XL pipeline.

She shouldn’t have to “wait for focus groups� to figure out where she stands on issues, O’Malley said recently.

But there’s a potential downside to too much piling on. Clinton’s record as secretary of State is, to some extent, the record of Democratic stewardship of the past eight years. Deep criticism of that tenure could come back to haunt Democrats later on.

*Explaining to Do: For all the talk about how Clinton used her private email server while at the State Department, the lingering question seems to be why she did it. The night offers her the chance to give a succinct explanation that lays concerns to rest.

Also, Republicans have been previewing a general election question for Clinton – what did she actually accomplish as secretary of State? With Sanders and the rest intent on pointing to holes in her record, Clinton has the opportunity now to supply lots of highlights.

Sanders has explaining to do, too, for an audience just now tuning in to his candidacy. He runs ahead of Clinton in state polls of New Hampshire, which is next door to his base in Vermont, but outside of devoted followers on the left-end of the party are a lot of Americans who aren’t quite sure what he means when he says he’s a Democratic socialist. He also has to defend his record on gun control, which includes opposition to the Brady Bill, which imposed background checks on gun purchasers, and support for a law absolving gun dealers and manufacturers of responsibility for crimes committed with the weapons they supply.

But most of the pressure is on Clinton. If Biden is watching the debate from the sidelines for signs of vulnerability, she’ll have to be deft at answering the tough questions while also making the case for a Clinton presidency.

*Breakout Capacity: The three lesser-known candidates on the stage have an opportunity to make themselves known outside their smaller circles of influence. Can any of them make use of the spotlight -- the way their Republican peers have done in the GOP debates – to break out of the ranks of the unknowns? Or will their days be numbered as the race grows more expensive and support drifts to the race’s leaders?

*Not Being Obama: Whoever they present themselves to be, each of the candidates needs to make one thing perfectly clear – they are not clones of the sitting president. After seven years of constant Obama critiques, candidates may decide they’re better off distancing themselves from their fellow Democrat and focusing instead on what they will do differently.

The leading candidates disagree with Obama’s trade policies and think he should have done more to change U.S. immigration policy. They say they would find a better way to work with the Republican leaders in Congress.

On the other hand, winning the presidency as a Democrat involves recreating the Obama coalition of young voters, women and minorities. A significant majority of Democrats say they want to see the next president largely continue Obama’s policies. Too harsh an approach to the president’s record could cause trouble.

For news about President Obama and the Obama administration, follow me on Twitter: @cparsons