They’re standing center stage tonight in the first of two second-round Democratic primary debates.

But any fireworks between Sens. Elizabeth Warren and Bernie Sanders may be muted – even as they battle for the progressive heart of the Democratic Party.

“Bernie and I have been friends forever. And certainly long before I ever got into politics of any kind,” Warren told Fox News over the weekend. And on the eve of the debate, she told reporters “look, I’m not against anybody.”

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Instead, the often-allied senators are more likely to face attacks on their far-reaching agenda from the field's lesser-known, albeit feisty, center-left candidates who have warned that such ideas are costly, "impossible" and a political gift to President Trump.

If the apparent truce between Warren and Sanders can hold, a central test will be whether they can effectively parry those attacks.

Thanks in part to her well-received performance in last month’s first round of debates, the populist Democratic senator from Massachusetts is now basically tied for second place with her fellow progressive senator from Vermont in the latest national and early primary and caucus voting state polling, behind front-runner former Vice President Joe Biden.

And Warren’s become a grassroots fundraising machine, announcing last week that she joined Sanders as the only two candidates in the record field of two-dozen contenders who’ve hauled in individual contributions from more than 1 million donors.

Warren told Fox News that she sees the debate as “a chance to talk about my vision for what’s broken in America, that we have a country that’s working better and better for a thinner and thinner slice at the top and not working for everyone else. And I hope other candidates are going to talk about their vision.”

With Warren now encroaching on Sanders' once-large lead in the fight for the party’s left, one might expect Sanders to take aim at his rival. But that doesn’t appear to be his strategy.

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“Intelligence” is what Sanders – an independent who’s making his second straight run for the White House – said recently when asked what he expected from sharing the center stage with Warren.

And Sanders campaign manager Faiz Shakir predicted that his boss and Warren will “end up agreeing” on most policy issues.

But Mo Elleithee, the founding executive director of Georgetown University's Institute of Politics and Public Service and a Fox News contributor, emphasized that “Bernie’s campaign is facing some real challenges. He’s still a top tier candidate but he has been on a downward trajectory while Warren’s been on an upward trajectory.”

“He’s got to do something to stand out. He’s got to do something to regain his footing,” he added. “Up 'till now his favorite foil has been Biden. But Biden’s not going to be on that stage.”

Elleithee, a senior spokesman for Hillary Clinton’s 2008 presidential campaign who later served as communications director for the Democratic National Committee, predicted Sanders may target the former vice president “even though they don’t share the same stage.”

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Both Sanders and Warren hope to use their time in front of a national audience in prime time to contrast their “bold” progressive agendas with the more incremental approach advocated by Biden.

“I want to see us make real change in this country structural change. In this country. And I think we can do it. We can do it together,” Warren stressed.

If Warren and Sanders do come under attack, the incoming fire will likely come from some of the more centrist candidates sharing tonight’s stage – including former Colorado Gov. John Hickenlooper and former Rep. John Delaney of Maryland.

On the eve of the debate, Hickenlooper tweeted that Warren “has some big ideas that have an even bigger cost. We proved in Colorado that you don't need big, expensive government programs to achieve progressive goals. Let’s talk about it tomorrow night.”

And hours before the showdown, Delaney gave Fox News a preview of his plan of attack.

“I’m running on real solutions, not impossible promises. I will point out what other people are putting forth on possible promises,” he emphasized. "If we run on some of these things, we're going to lose to Donald Trump."

Rep. Tim Ryan of Ohio, another longer-shot for the nomination, also previewed his line of attack against Warren and Sanders.

Asked about "Medicare-for-all," he told Fox News hours before the debate that “I don’t think you can square taking private insurance away… That’s a bad idea and it’s really bad politics too.”

Warren said she's ready to defend her turf.

Asked if she’s prepared to return fire, Warren told Fox News: “I’m always ready… I am ready to defend all of my plans because they’re important and they’re things that touched the lives of people all across this country.”

But she stressed that going negative during the debates is “not the point. This is really about a chance for Democrats to tell America their affirmative vision of what we should be doing in this country.”

And Warren appeared to take a dig at some of her rivals, noting, “I don’t think it’s enough to say ‘not Trump.’ I think we need to come forward and say ‘here how we see it. Here’s how we think it’s broken and here’s how we intend to fix it.’”

For Hickenlooper, Delaney, Ryan and the other lower-tier candidates on the stage tonight -- Montana Gov. Steve Bullock and best-selling spiritual author Marianne Williamson -- tonight’s showdown might be their last best chance to make a splash. With the Democratic National Committee significantly raising the thresholds for the third and fourth rounds of debates, more than half the current field might not qualify.

Two other White House hopefuls on tonight’s stage – South Bend, Indiana Mayor Pete Buttigieg and former Rep. Beto O’Rourke of Texas – have already qualified for the upcoming rounds of debates. And Sen. Amy Klobuchar of Minnesota is closing in on reaching the thresholds.

O’Rourke surged after jumping into the race in early March, but he’s seen his poll numbers and his fundraising figures sink in recent months. This debate gives him a chance to make a second impression.

Buttigieg’s seen his poll numbers plateau in recent weeks, though he hauled in more campaign cash than any other Democratic contender during the April-June second quarter of fundraising.

Pointing to the potential debate stage faceoff between the two candidates, Elleithee predicted that “if I’m Buttigieg, I’m going to ignore Beto. And If I’m Beto, I’m going to try and take Buttigieg out.”