The spot ends with Sanders and Ocasio-Cortez clasping hands.

The phrase “I'm willing to fight for someone I don’t know” has gone viral, with progressive activists, Sanders fans, and even some Hollywood stars such as Don Cheadle and Piper Perabo tweeting the message of solidarity.

At the event, Sanders also said, "Are you willing to fight for young people drowning in student debt even if you are not? Are you willing to fight to ensure that every American has health care as a human right even if you have good health care? Are you willing to fight for frightened immigrant neighbors even if you are native-born?"

It's an addition to Sanders’ stump speech and notable for sending an implicit message to voters who are not yet supporting him. The Vermont senator has struggled to expand his base so far in the presidential race.

The Sanders campaign estimated about 26,000 people attended the Saturday event, making it the biggest rally of any Democratic candidate this year. It was Sanders' first rally since he suffered a heart attack at the beginning of the month.

Sanders’ spot also highlights a diverse array of rallygoers. His aides have said the endorsement from Ocasio-Cortez — as well as one from Rep. Ilhan Omar — demonstrates the multiracial working-class coalition that he is building.

In addition to the first paid ad featuring Ocasio-Cortez, the Sanders campaign unveiled this weekend a video featuring her endorsement of Sanders. It has nearly 3 million views on Twitter.