Thousands of Bernie Sanders supporters swarmed across New York's iconic Washington Square Park Wednesday night ahead of a make-or-break primary in the state on April 19.

The rally — one of the largest yet for the Vermont senator — drew some 27,000 people who filled the park and swarmed the surrounding streets of Greenwich Village, according to campaign estimates. The pre-speech line-up incorporated a slate of New York celebrities and bands including actress Rosario Dawson, filmmaker Spike Lee, and the indie-rock band Vampire Weekend, who performed at dusk at the base of the park's recognizable marble arch.

Sanders, who began his stump speech by noting he and wife, Jane, had left New York when they were kids, reiterated the campaign's belief that a win in New York would propel him to the White House.

He couched that statement later by saying that the state's closed primary system, which precludes independents and other unaffiliated voters from casting ballots, would provide a challenge to the campaign, but said that high voter turnout was the key to victory.

"I think we've got a surprise for the establishment," Sanders said to thunderous cheers. "I think that if we have a large voter turnout on Tuesday, we're gonna win this thing!"

Sanders surrogates also received rapturous applause for their introductory speeches, although healthcare activist and physician Paul Song caught some controversy when he referred to "corporate Democratic whores," while advocating for Sanders's single-payer healthcare system.

"Medicare-for-all will never happen if we continue to elect corporate Democratic whores who are beholden to big pharma and the private insurance industry instead of us," Song said Wednesday night.

The comments sparked criticism from Clinton supporters and the campaign's communications director Jennifer Palmieri, who called the language "distressing."

Song walked later back the comments in a tweet on Wednesday, while the Sanders campaign also tweeted Thursday:"Dr. Song's comment was inappropriate and insensitive. There's no room for language like that in our political discourse."

DemocraticWhores has been trending in the United States all of Thursday morning.

Wednesday's rally could provide Sanders with much-needed momentum as the senator prepares to dip out of campaign trail for a couple of days to head to the Vatican, where he'll sit in on a conference on social justice and environmental issues. The event also took place the evening before a much anticipated Democratic debate with rival Hillary Clinton on Thursday night.

Sanders will fly to Rome directly after the CNN-hosted debate, in a highly unusual move just days before a must-win primary on Tuesday. Clinton is currently polling between 10 and 14 points ahead of Sanders in the state. As late as February and March, Clinton led Sanders by as much as 21 points. Sanders Campaign Manager Jeff Weaver defended the decision to make the Vatican trip so close to the pivotal primary, telling Bloomberg that the invitation is a "can't miss opportunity," and "some things are more important than politics."

"When you get invited by the Vatican, I think you go," Weaver said.

Follow Liz Fields on Twitter: @lianzifields