If Bernie Sanders is to win the Democratic presidential nomination — and he is behind — he has to step up and sharpen his case against Joe Biden, which he did at a rally in Grant Park on Saturday in a drive to get out the vote for the March 17 Illinois primary.

Sanders’ immediate problem is not President Donald Trump. Democrats of all stripes are united in wanting to defeat him in November. Sanders has to deal first with the surging Biden, who revived his near-dead campaign with decisive wins in South Carolina and Super Tuesday.

“Now that the election, the primary nomination process is coming down to two people, it is important for the American people, the people of Illinois, to understand the differences between us in terms of our record, in terms of our vision for the future,” Sanders told the adoring, youthful crowd.

“Joe Biden and I are friends. I have known him for many years, but we have different records. We have a different vision. The American people will hear about it,” Sanders said.

The Vermont senator, an independent, then hit Biden, a former Delaware senator and ex-vice president, on his votes for the Iraq war, the Wall Street bailout, the abortion-related Hyde Amendment and trade agreements, including NAFTA.

“Those trade agreements were a disaster. Joe was wrong. I was right.”

Sanders also slammed Biden for raising money from billionaires and accepting the support of super PACs. “I don’t have a super PAC. We don’t want a super PAC. We don’t need a super PAC. I don’t go to billionaire’s homes for campaign contributions,” he said.

Those issues did not resonate and energize the thousands under a sunny sky in Grant Park as much as Sanders’ pledge for “Medicare for All” and his promise to “cancel all student debt in America,” though he never explains how exactly he would convince Congress to give everyone the break.

Sanders, not one to usually pander to a crowd, broke his self-imposed rule when he talked about how, if president, he would issue an executive order — meaning he would not need congressional approval — to make “marijuana legal in every state in this country.”

“And I invite you to the ceremony when we do just that,” he said to a sustained cheer. “Matter of fact, maybe we will do it right here in Grant Park.”

What Sanders did not do on his visit was make any mention that he actually lived in Chicago’s Hyde Park community for years. Sanders attended Brooklyn College for one year before transferring to the University of Chicago, where he earned his undergraduate degree in 1964.

It was a curious omission because on social media, Sanders was throwing a spotlight on his Chicago connections, saying on Facebook, “My years here in Chicago significantly shaped my life. They gave me the opportunity to become involved in the civil rights movement, in the labor movement, in the peace movement. That is the struggle we carry on today.”

Indeed, Sanders’ Chicago supporters are part of the movement he created in the wake of his 2016 presidential campaign. He was introduced by Rep. Jesus “Chuy” Garcia, D-Ill., the most prominent Illinois elected official backing Sanders. The two go back years, when Sanders endorsed Garcia during his ill-fated mayoral run against Rahm Emanuel.

Garcia introduced Sanders, calling him the “transformational leader that America needs” as loudspeakers blared the song “Power to the People.”

Sanders filled the area in front of the Petrillo Music Shell; an aerial view photo taken when Sanders was speaking showed that he filled some but certainly not all of Grant Park.

Sanders wedged in Chicago between stops in Michigan — hitting Dearborn in the morning and Flint at night. Michigan is one of six states voting this Tuesday, along with Idaho, Mississippi, Missouri, North Dakota and Washington.

In Illinois, Biden’s support comes from much of the Democratic political establishment. Sanders has the anti-establishment Democrats.

“What the political revolution is about is rethinking how we look at politics,” Sanders said, “I know many of you have friends who have given up on the political process ... and we got to tell those people, stop complaining. Get involved in the political process.”

The Sanders campaign is cancelling a rally scheduled for Tuesday in Rockford. Giving up an Illinois rally to work states voting that day makes sense. If Sanders does not do well in delegate-rich Michigan and the other March 10 states, his path to the nomination is narrowed.

Illinois and the March 17 states may be Sanders’ last stand.