But after two poor debate showings, and a Super Tuesday rout, Bloomberg dropped out of the race Wednesday . That and the recent withdrawals from the race for Mayor Pete Buttigieg and Sen. Amy Klobuchar, D-Minn., have made Biden the only real anti-Bernie choice.

Another reason for Biden's resurgence was former New York City Mayor Mike Bloomberg's campaign. Bloomberg's entrance into the race produced the unintended consequence of splitting the moderate vote and thus boosting the odds for more extreme Sanders . Bloomberg also seemed to be the living embodiment of everything Sanders has been saying for years about billionaires having too much influence in U.S. politics.

So far, the last-minute endorsements and pressure on rival candidates to withdraw from the race have worked like a charm. Biden now has the lead in the Democratic delegate count , and he's once again the front-runner for the party's presidential nomination.

After Biden's disappointing showings in the Iowa caucuses and the New Hampshire primary, a clearly rattled Democratic Party establishment hit the panic button and decided to coalesce around the man they saw as the most viable alternative to Sanders.

Specifically, fear of Sen. Bernie Sanders, the Vermont independent, winning the Democratic presidential nomination, knocking establishment Democrats out of power, presenting a socialist message to the voters, and handing President Donald Trump a decisive general election victory.

There's one word to explain former Vice President Joe Biden's big turnaround from what looked like a dead-in-the-water presidential campaign just a week ago.

Meanwhile, Sanders has been hit by weak turnout among the younger voters who are the life's blood of his campaign. You can't beat the establishment if you don't show up to the playing field.

But here's the big question: Will that be enough for Biden to win the nomination and the general election?

It sure seems like the answer to the first part of that question is "yes." Biden has clinched priceless momentum in the Democratic race and he may even pull off the huge surprise of winning enough delegates to avoid the "brokered convention" so many experts have been predicting for weeks.

Meanwhile, Sanders has been hit by weak turnout among the younger voters who are the life's blood of his campaign. You can't beat the establishment if you don't show up to the playing field.

Fear accounts for another specific reason for Biden's comeback, and that is the coronavirus issue. Exit polls showed that Democrat Super Tuesday voters who rated the coronavirus as an important factor in their voting choice overwhelmingly voted for Biden.

If the virus continues to spread and wreak havoc with the economy, there's no doubt Trump will continue to see his poll numbers suffer. But Biden will still have to project an image of a leader who is capable of tackling the problem head-on. If all he does is bash Trump over his handling of the coronavirus impact, he could fall flat on his face when he's asked what he would do differently. Those who think the coronavirus effect is only a wild card or threat to Trump are mistaken.

That's just one way the general election contest against Trump is an entirely different matter.

At some point, Biden will need to make the case for himself as a candidate for president. Focusing on the fact that he's not the other candidate many voters dislike or fear more won't be enough.

With more of a spotlight on him than ever, Biden will have to overcome his long history of losing his luster once voters get a better look at him. In the past, Biden hasn't been able to get enough Democratic primary voters in any state to support him. It's truly a big deal that Biden has finally cleared that hurdle with his own party's voters. Now, he needs to prove he can do that with the general public.

Whether he can requires asking whether Biden has really changed anything about himself or his campaign. His debate performances have been less than scintillating and he continues to make gaffes that instantly go viral on social media.

Then there's that issue of weak turnout among younger voters. That weak turnout helped Biden vanquish Sanders in many states Tuesday, but if the trend keeps up through November it will likely be a boon for Trump.

The conventional wisdom has been that the big danger for any establishment Democrat defeating Sanders is that Bernie's younger supporters won't transfer their enthusiasm to an establishment nominee. But the dangers are even greater for Democrats if younger voters continue to show little enthusiasm for any candidate.

Fear can take you a long way in American politics. But Joe Biden will need to get more voters to actually believe in him if he wants to become president.

Jake Novak is a political and economic analyst at Jake Novak News and former CNBC TV producer. You can follow him on Twitter @jakejakeny.