The first joint televised appearance by the eight Democratic presidential candidates tonight is likely to help define both the party’s overall approach toward an eminently winnable election and the differences among them.

The encounter in South Carolina (live on MSNBC) may also provide an early clue whether the top contenders can withstand the pressure of a highly competitive race and avoid the negativism and harsh rhetoric such contests often produce.

This pressure is mainly on the early front-runners – Sens. Hillary Rodham Clinton and Barack Obama – to score political points at each other’s expense. But there may be an even greater incentive for them to keep their approach positive and forward-looking.

Meanwhile, their rivals, headed by 2004 vice-presidential nominee John Edwards, may feel they need to go after Clinton and Obama to make their candidacies seem more relevant and counter the premature wisdom that it’s a two-candidate race.

That temptation might be greatest for Edwards. The former North Carolina senator has been focusing on Iowa, Nevada, New Hampshire and South Carolina, the four states likely to begin the nominating race next winter, where he presumably must beat one or both main rivals. To do so, he has tried to cast himself as the leading anti-Iraq candidate by repudiating his 2002 vote in favor of action to topple Saddam Hussein.

But Edwards made his name four years ago by avoiding the rat-a-tat-tat that led to the self-destruction of some rivals, and he might pay a big price if he abandons that approach.

Similarly, Clinton and Obama risk losing more than they might gain if they try to score debating points at the expense of each other. Clinton, whose lead has dwindled in many polls but who remains favored by party insiders and pundits, needs to avoid anything that accentuates her image as a sharp-tongued partisan.

Obama, who has succeeded so far in portraying himself as a future-oriented candidate, needs to show there is more to him than a charismatic personality promising change. Democratic Party activists, the main players at this stage of the contest, tend to like candidates with detailed policy prescriptions. He has started to produce some but it may be difficult for him to do so and differentiate himself from Clinton without playing into the party’s negative “big spender” image.

A prime focus will be on what they and the others say about Iraq. Clinton voted to authorize the war and has not repudiated her vote, though she has criticized how President Bush has managed the war. Obama has said since 2002 he would have opposed the resolution authorizing an attack had he been in the Senate but, as National Journal noted last week, he has not been totally consistent in votes on funding the war and withdrawing U.S. troops.

While his position may appeal more to the heavy Democratic majority opposed to the war, a February poll showed most Democrats don’t want Clinton to repudiate her vote. It remains unclear what role these past differences will play in a campaign likely to focus on how and when to end the U.S. role in Iraq, an area where the two rivals are in substantial agreement.

In a sense, these televised encounters – they’re not really debates – provide the best chances for candidates such as Sens. Chris Dodd of Connecticut and Joe Biden of Delaware, Gov. Bill Richardson of New Mexico, former Alaska Sen. Mike Gravel and Rep. Dennis Kucinich of Ohio to make a showing that attracts additional political backing and financial support and enables them to climb into the top tier of candidates.

Kucinich, who has introduced an impeachment resolution against Vice President Dick Cheney and favors immediate withdrawal from Iraq, will establish the left flank of the debate, but it seems unlikely most others will follow him. Biden, Dodd and Richardson need to find a way to show they are experienced and intelligent public officials without trying to tear down the leaders.

Inevitably, the candidates will spend some time trashing Bush and the way he has mishandled issues from Iraq to the Justice Department. Another obvious target will be Tuesday’s comments by GOP hopeful Rudy Giuliani that a Democratic victory would put the country “back on defense” in the war on terror.

But the candidates ought not to forget the need to give Democrats, and ultimately the broader electorate, a positive reason to make one of them the next president.