The American Prospect’s Robert Kuttner has an insightful take on “What to Watch for in Tuesday’s Debate” in terms of Clinton’s strategy:

Clinton needs to get out of a self-infecting cycle of bad publicity, in which everything she does is dismissed as calculating and contrived, even when it represents creative movement on issues. Sanders merely needs to take care to come across as fighting for the forgotten American on the issues, as he nearly always does, but not too radical in his personal style.
In the past few weeks, Clinton has made several dramatic moves in Sanders’s direction. She has broken with the administration on the Trans-Pacific Partnership trade deal, on the Keystone Pipeline, and on the so-called Cadillac Tax on high quality health plans (she is for repeal; the White House is not). She is out-flanking Sanders to the left on gun control, and she is at least as comfortable talking about race.
…In the inside game, Clinton needs to persuade the activists associated with the Democratic Party, especially the labor movement, that she can be as much their champion as Sanders can. She needs to reassure her own core supporters (who might be tempted to defect to Biden) that her candidacy is not fatally damaged by recent missteps.
..Clinton, in short, is necessarily playing a much more complex game than Sanders. Much of her posture is directed at a potential candidate who will not be on stage–Joe Biden. A great deal of her positioning is aimed not just at Sanders, but at dissuading Biden from getting into the race.

Clinton will have to provide clear answers — and good soundbites — in response to the badgering she will receive about her emails, discrediting the accusations as baseless, politicized complaints, without seeming arrogantly dismissive. A challenge for her, and for all of the candidates, is not to bristle when under attack.
As for Sanders, Joan Walsh notes at The Nation:

…Sanders has improved his rhetoric and his outreach since those early clashes. He hired Symone Sanders, a young African-American activist on issues of mass incarceration and racial justice, away from Public Citizen to be his communications director. And where he once sounded as though he believed the achievement of genuine economic justice would lead automatically to racial justice, he now routinely talks about dismantling the incarceration state and other measures specifically designed to reverse black disadvantage.
On guns, Sanders has riled activists with a handful of votes against gun regulation. He voted against the 1993 Brady Bill, to allow weapons in national parks and checked baggage on Amtrak, and to offer gun manufacturers immunity against suits by gun victims. In condolence remarks after the mass shootings in Charleston, South Carolina last summer, Sanders didn’t mention the issue of guns.
But Sanders has improved his rhetoric and his outreach since those early clashes. He hired Symone Sanders, a young African-American activist on issues of mass incarceration and racial justice, away from Public Citizen to be his communications director. And where he once sounded as though he believed the achievement of genuine economic justice would lead automatically to racial justice, he now routinely talks about dismantling the incarceration state and other measures specifically designed to reverse black disadvantage.
…The big question for Sanders is whether he can put together an electoral coalition to get the nomination, and win next November. On that score, the debate can’t help but help him. Sanders still polls dismally among African-Americans; in a recent YouGov poll he got 8 percent of their votes; in a South Carolina poll released Monday (that’s the first primary state in which the black vote will be significant), he was at 4 percent. But a lot of that has to do with his being much less known to black voters than Clinton or Vice President Joe Biden. The first debate gives him a chance to bring his appeal to a mass audience.

Many political observers have expressed skepticism about Biden’s chances, should he eventually decide to run. A new Reuters poll indicates, however, that a Biden candidacy would have substantial support, even though he won’t be in the first Democratic debate:

Biden will not be there, but 48 percent of Democrats surveyed in the Reuters poll wish he were a candidate, compared with 30 percent who said he should stay out. Independents were split on the question, with 36 percent saying Biden should stay in and an equal share believing otherwise.
But support for Biden’s entry into the race does not translate into equal passion for his candidacy. Just 17 percent of those surveyed said Biden would be their first choice, while 46 percent would back Clinton. Biden would also run behind Sanders, who remains the favorite of one fourth of Democrats surveyed.

Lawrence Lessig, the crowd-funded academic who is focused on one issue — campaign finance reform, also will not be at the debate, since he has been polling below one percent.
Regarding the longer-shot candidates, who will all be looking for a possible “Fiorini moment,” Catherine Lucey and Ken Thomas of the AP quote former MD Governor Martin O’Malley on the challenge he faces tonight:

“This will really be the first time that nationally voters see that there’s more than one alternative to this year’s inevitable front-runner, Secretary Clinton,” O’Malley said.
“It’s a very, very important opportunity for me to not only present my vision for where the country should head, but also 15 years of executive experience, actually accomplishing the progressive things some of the other candidates can only talk about,” he said.

Ed Kilgore adds at The Washington Monthly, “If there’s any justice, though, Martin O’Malley probably deserves a post-debate bump. The guy did things the way you’re supposed to, spending many obscure days and weeks in Iowa before anyone was even thinking about the presidential race.”
Rachel Weiner writes at the Washington Post that “If there’s a chance for a wild card on the stage at Tuesday’s lead-off Democratic debate, the smart money’s on former senator Jim Webb of Virginia.” Weiner quotes Webb campaign spokesman , who provides a clue as to the persona Webb will try to project: “We have the best candidate to deliver economic fairness, social justice and common sense foreign policy, unbought and unbossed by anyone.”
With respect to Lincoln Chafee, Lucey and Thomas write, “Expect Chafee, the former senator and governor from Rhode Island, to go after Clinton for her 2002 vote to authorize the war in Iraq. Chafee, at the time a Republican, opposed the invasion and he’s said Clinton’s support for the war, which she has more recently called a “mistake,” is at the center of his decision to run.”
Much to the relief of many Democrats, regardless of their candidate preferences, there will certainly be a vigorous debate, instead of a ‘coronation,’ which would surely be frowned upon by swing voters. The hope is that tonight’s forum will generate light, as well as heat — a big distinction from what has been going on in the GOP debates.