Elizabeth Warren has a history of embracing big challenges, from running against Scott Brown to taking on the nation’s biggest banks to feuding on Twitter with Donald Trump. But tonight in Philadelphia, she faces what might be her toughest task yet.

“There is a huge contrast between what we saw in Cleveland and what we’re gonna see in Philadelphia,” said Newton Mayor Setti Warren.

The Massachusetts delegation started the day on a upbeat note, with big-name Democrats stressing optimism and unity at a morning breakfast.

But lately, there’s been plenty of Democratic discord.

First, Hillary Clinton’s choice of Tim Kaine as her running mate sparked pushback from the party’s left wing. Then came a massive email leak which showed the party worked against Bernie Sanders, and led to Democratic National Committee chair Debbie Wasserman Schultz’s resignation.

“She absolutely did the right thing,” said Steve Grossman, a former chair of the DNC. He also thinks Sanders supporters will rally behind Clinton, and that Elizabeth Warren is uniquely qualified to give them a nudge.

Attorney General Maura Healey is equally bullish about Warren’s speech, partly because she thinks the Democratic divide has been overstated. “It’s okay: there’s passion, there’s energy. That’s the vibrancy of the democratic party,” she said. “Donald Trump and Mike Pence represent something very regrettable, very scary, in my opinion, about the direction of this country, and that’s where people need to focus.”

Of course, they’re Clinton supporters. State Senator Dan Wolf is a Sanders delegate. But he says he’s ready to shift gears, and that Warren can help convince any holdouts.

Still, the reluctance of other Sanders backers shouldn’t be underestimated. After today’s breakfast, Sanders loyalists met to discuss formally submitting his name for the nomination. The group’s leader says she’s still not convinced that Clinton will be the nominee.

Lesley Phillips, an elected delegate, said, “Who knows, Bernie may have been on the phone all night with our super delegates and 400 of them may have woken up with an attack of conscience.”

That shows just how tough Warren’s job tonight is. That Sanders backer said she wants Warren to rip the TPP trade deal, and talk tough about Wall Street. Whatever Warren does say, it’s clear the stakes are incredibly high.