Four of the five top-polling White House contenders will be on stage tonight, for what’s being billed as the main event in the first round of Democratic presidential primary debates.

But grabbing the most attention and generating the most friction will likely be the two veteran contenders standing center stage – former Vice President Joe Biden and Sen. Bernie Sanders of Vermont.

2020 DEMOCRATS CLASH ON MEDICARE FOR ALL, IMMIGRATION, IN FIRST DEBATE

Biden – the clear front-runner right now in the nomination race – is expected to face incoming fire from his rivals. Some of those jabs could come from Sanders, a vocal Biden critic who remains the former vice president’s closest competitor in the polls. And Sanders may also need to dodge arrows over his brand of democratic socialism, which some argue is moving the party too far to the left.

The former vice president’s name remained notably absent during Wednesday night’s first debate, when none of the 10 contenders – including frequent Biden critic Sen. Elizabeth Warren – took an opportunity to swipe at the front-runner. But it’s a sure bet that won’t be the case tonight.

“Everyone expects there to be enormous fireworks between Bernie Sanders and Joe Biden,” said Zac Petkanas, a Democratic consultant who served as director of rapid response for Hillary Clinton’s 2016 presidential campaign.

The progressive senator from Vermont – who battled eventual nominee Hillary Clinton in a testy 2016 Democratic nomination race – has jabbed at Biden the past two months over his vote in favor of the Iraq War, his support of free trade deals, his authoring of the 1994 crime bill that critics argue led to the disproportionate incarceration of minorities, his “middle ground” climate change proposal, and his fundraisers with top-dollar donors.

“The Sanders campaign has consistently looked to Biden to act as a foil,” Petkanas said.

LONG-SHOTS GRAB SPOTLIGHT AT FIRST DEBATE

Ahead of the second debate, Sanders gave no hint he’d launch broadsides against Biden during the debate showdown.

“We look forward to a serious debate about the serious issues facing the working families in this country,” Sanders told reporters. “I’m sure it’s going to be a hard-hitting issue-oriented debate and that’s what the American people want.”

The friction between Biden and Sanders symbolizes the Democratic Party’s key ideological fight between the pragmatism during President Barack Obama’s administration and Sanders’ push to move the party leftward toward democratic socialism.

It’s not just Sanders who’s been willing to call out the former vice president. The two candidates standing aside Biden and Sanders have nipped at the front-runner. Sen. Kamala Harris of California criticized Biden over the crime bill and 37-year-old South Bend, Indiana Mayor Pete Buttigieg has repeatedly highlighted that it’s time for “new leadership” and a generational change, an indirect snipe at the 76-year old former vice president.

Biden’s coming off the toughest week of his White House bid. His unforced error of recalling how he found common ground with segregationist senators to spotlight his ability to get things done has offered his rivals new ammunition. It’s likely to come up in the debate.

To date, Biden’s refrained from returning fire.

Asked earlier this month if he’d stay restrained on the debate stage, Biden told Fox News “I think the worst thing we could do is get into a match where we’re going after each other in the Democratic Party. So I’m going to try my best not to be negative relative to my opponents.”

While he vowed not “to go down to anybody else’s level when they start attacks,” Biden added he would “respond to assertions” to “set the facts straight.”

LATEST 2020 PRESIDENTIAL CAMPAIGN DEVELOPMENTS FROM FOX NEWS

And a day ahead of the showdown, Biden advisers predicted their candidate would go high even if his rivals went low and told Fox News he'd stick to the issues.

For others on tonight’s debate stage, there could be a downside to taking on the front-runner.

Petkanas emphasized that “the big danger” the candidates face is how to “engage with other candidates and show differences and contrasts without damaging themselves and allow another candidate who’s staying above the fray to emerge victorious at the end of this long slog.”

The debate also offers an opportunity for Harris and Buttigieg – the other top tier candidates on the stage. Harris – a former California attorney general and San Francisco district attorney – is expected to make the case to a national audience that thanks to her prosecutorial skills, she’s the best Democratic contender to go toe to toe with Republican President Donald Trump.

For Buttigieg, the showdown comes less than two weeks after the fatal shooting of a black man by a white South Bend police officer. The incident pulled Buttigieg off the campaign trail and the mayor’s been criticized by both minority leaders and the city’s police union for his handling of the incident.

“There’s a real question on whether he should be on that debate stage or back in South Bend trying to mend fences with the community upset about the police shooting. He really needs to do something positive,” argued Democratic strategist and presidential campaign veteran Jamal Simmons.

Joining Biden, Sanders, Harris, and Buttigeig on the debate stage are Sens. Kirsten Gillibrand of New York and Michael Bennet of Colorado, former Colorado Gov. John Hickenlooper, Rep. Eric Swalwell of California, entrepreneur Andrew Yang and best-selling spiritual author Marianne Williamson.

The ideological divide evident in the first debate – with Sen. Amy Klobuchar of Minnesota, former Rep. John Delaney of Maryland, and Rep. Tim Ryan of Ohio pushing back against a mostly progressive roster of candidates led by Warren – will likely play out again tonight, with Biden, Hickenlooper and Bennet taking on the progressives led by Sanders.

For the lower-tier candidates – tonight’s mission is to make a splash.

Two of those one-percenters did break through on Wednesday night – by getting aggressive. Both former Housing and Urban Development Secretary Julian Castro and New York City Mayor Bill de Blasio grabbed the spotlight by tangling with former Rep. Beto O’Rourke of Texas. Some of the candidates standing on the ends tonight may try to follow in their footsteps.

Former Democratic National Committee chair Donna Brazile, who’s a Fox News contributor, emphasized on ‘America’s Newsroom’ that “these candidates have to figure out a way to basically stand out. And, to also draw a contrast with each other.”

Veteran Democratic consultant and communications strategist Lynda Tran highlighted that the lower-tier candidates “should focus on sharing their vision for the country — in real people, relatable terms — and showing both the seriousness they would bring to the Oval Office and some of their personality.”

But Petkanas warns that might be easier said than done.

“The debate presents a real opportunity for [lower-tier candidates], but also challenges. Will they be able to inject themselves into what’s being billed as the ‘Bernie-Biden’ show and will they be able to do it in a smart way,” he questioned. “There’s probably going to be very little oxygen for anything besides Bernie and Biden.”

One of those longer shots is Yang – whose pledge to give every adult American a universal basic income of $1,000 a month is grabbing some buzz. He told Fox News on Wednesday he’s “got half a dozen planned jokes.”

And noting his position on the stage, he quipped that the “only thing standing between me and Biden is Buttigieg.”