Editor's Note: (David Axelrod, a senior CNN political commentator and host of "The Axe Files," was senior adviser to President Barack Obama and chief strategist for the 2008 and 2012 Obama presidential campaigns. The opinions expressed in this commentary are his. View more opinion articles on CNN.)

(CNN) Bernie Sanders has a lot to be proud of.

Five years ago, the shambling, stoop-shouldered septuagenarian launched a quixotic campaign for the presidency and pushed Hillary Clinton -- a brand name in Democratic politics -- to the limit.

This year, he did it again, defying those who said the Vermont senator's day had passed --even overcoming a mid-campaign heart attack -- to forge another potent challenge to the Democratic establishment.

Along the way, he shined a bright light on the weaknesses in the country's social safety net (which have been further bared by the current Covid-19 crisis); the yawning and growing inequality in our economy; the mounting burden of student debt; the corrupting power of special interests; and the mortal threat of climate change.

It is a message that galvanized millions, particularly the young. They are demanding wholesale changes to address what they see as shameful systemic failures that conspire against working class Americans and the poor.

Sanders, a self-styled Democratic Socialist who has never actually claimed membership in the Democratic Party, outlasted all but former Vice President Joe Biden out of a field of more than two dozen.

But at the almost certain risk of bringing down the wrath of the acid-tweeting Bernie Bros: It's over, Senator. It's time to go.

Yes, the campaign is in a weird state of suspended animation as America grapples with a horrific global pandemic. Yes, 23 states have yet to vote. But the math is immutable. Barring an act of God -- which admittedly may not be the best turn of phrase at the moment -- Biden will be the nominee of the Democratic Party.

Biden holds a 317-delegate lead over Sanders. That means Sanders, who trails Biden by 20 points in national polling, would have to win 60% of the remaining delegates to overtake Biden.

Even if Sanders got on an unforeseen roll, such a rally would be a near impossibility. Unlike the winner-take-all Social Darwinists over in the Republican Party, Democrats award delegates proportionately, according to the popular vote. As long as you clear 15% in a state, you get delegates -- which makes a lead the size of Biden's insurmountable.

Way back in...well, a month ago, Sanders said he would not continue if the nomination were out of reach.

"I'm not a masochist who wants to stay in the race that can't be won," Sanders told George Stephanopoulos on ABC's "This Week."

Now he must know that he is in a race that can't be won.

His most ardent supporters argue that Sanders should remain to continue to give voice to his progressive agenda and leverage his support to influence the party platform. But he already has done that to the extent he can, compelling Biden and other Democrats to move, even if not all the way, in Bernie's direction on a variety of issues, such as free tuition for many young Americans at public colleges and universities.

There will be no more debates. And, given the gravity of the pandemic and resulting economic coma, there will be virtually no opportunity to break through the news in any meaningful way.

To linger in the race, moreover, only delays and complicates a coming together around the mission of defeating President Donald Trump, whose reelection Sanders has described as "an existential threat."

In a recent ABC News-Washington Post poll, an astonishing 15% of Sanders' voters said they would support Trump in November. To Sanders, who bristles at the suggestion that his prolonged and bitter challenge to Clinton in 2016 helped facilitate Trump's election, this should be a concerning number.

With Sanders idling his engine in Burlington, Vermont, his most zealous supporters -- and Trump-inspired bots posing as Bernie supporters -- continue to trash Biden online in terms remarkably similar to the president's own "Sleepy Joe" memes.

Trump, himself, has become Sanders' greatest champion, accusing Democrats of "cheating" Sanders out of the nomination (this is when he is not calling Sanders "Crazy Bernie"). Team Trump is eager to stir this pot and drive alienated young Berniacs away from the polls in November, as they did in 2016.

Unlike his bitter breach with Clinton, for whom he campaigned but with little enthusiasm, Sanders clearly has affection for Biden. Whatever their differences, they pale in comparison to the gulf between them and Trump.

Politics seems well beside the point at a time when the country is focused on just one thing, and that is the brutal effects of a raging pandemic and the draconian measures necessary to blunt it.

Yet there will be an election this fall and the crisis we face has only underscored the consequences of that choice. Given the magnitude of the moment, that is a message Sanders can deliver with characteristic passion to his faithful supporters.

A strong, early statement of support by Sanders for Biden, and an unambiguous call to unity and action in this moment, would be valuable and important. It also would unleash the Democratic National Committee to put its resources fully behind Biden as the nominee.

You have a lot to be proud of, Senator.

Now it's time to go.