On Saturday, more than 25,000 Bernie Sanders supporters gathered in Queensbridge Park, New York.

As BuzzFeed News reported, this was “the largest crowd in support of any Democratic campaign in the primary to date.”

A lineup of prominent Sanders supporters took the stage. Notably, filmmaker Michael Moore and Democratic Representative Alexandria Ocasio-Cortez rallied the enthusiastic crowd.

Then Sanders took the stage, to AC/DC’s “Back in Black.”

“I am happy to report to you that I am more than ready — more ready than ever to carry on with you the epic struggle that we face today,” the Vermont senator said. “To put it bluntly, I am back.”

Sanders came “back” following a health scare, and with endorsements from Ocasio-Cortez, and Ilhan Omar.

The high-profile endorsement came at just the right time for the Vermont senator, as some were beginning to doubt his campaign, and in the aftermath of a strong debate performance.

The massive rally has reassured even those skeptical of Sanders’ chances, it seems, breathing new life into his campaign.

Now, even some Republicans are alarmed.

According to a new report from The Daily Beast, Republican strategists are growing increasingly worried about Sanders. With Ocasio-Cortez in his corner, and with a grassroots movement behind him, Sanders seems to have spooked the GOP.

“Anyone who counted out Bernie Sanders is going to be wrong,” former Trump campaign adviser Michael Caputo said.

“I see Bernie as far more formidable now than he ever was,” he continued, observing that Sanders has, with his candidacy, managed to mount a challenge not just to the Republican Party, but to the Democratic Party as well.

Sen. Bernie Sanders: "If you are willing to love, if you are willing to fight for a government of compassion and justice and decency... together we will transform this country." pic.twitter.com/GcOTOxrpu8 — The Hill (@thehill) October 20, 2019

According to veteran Republican strategist Doug Heye, even though crowd sizes alone don’t win races, “you can’t argue with the crowd size.”

According to Heye, the Sanders campaign now needs to demonstrate what states the Vermont senator can win, and beat Massachusetts Senator Elizabeth Warren, who has positioned herself as “Bernie-lite.”

As the publication notes, although famously obsessed with crowd sizes, President Donald Trump has remained silent about Sanders’ rally, focusing his Twitter attacks on former Secretary of State Hillary Clinton.

Clinton is currently feuding with Hawaii Representative Tulsi Gabbard, accusing her of being a Russian asset.

While Trump may have decided to ignore Sanders, the Republican Party has not. Via social media, the GOP is attacking Sanders and his progressive allies.

Our billionaire president who coddles the elite and betrays the working class is going to lose, and we are going to defeat him — Faiz (@fshakir) October 19, 2019

According to polls, Sanders would be a strong candidate in the general election. However, to even have a shot at challenging Trump, he has to win the Democratic primary first, where he appears to be a weaker candidate.

Warren and former Vice President Joe Biden are both ahead of Sanders in most polls.