Julian Zelizer is a professor of history and public affairs at Princeton University and co-author with Kevin Kruse of the new book " Fault Lines: A History of the United States Since 1974 ." The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) Former Vice President Joe Biden is not the right Democratic candidate for 2020. And yet, he may soon enter the Democratic race.

Thus far, what has been most remarkable is how diverse the field is shaping up to be. This is not your traditional slate of Democratic candidates. Kamala Harris, Amy Klobuchar, Cory Booker, Julian Castro, Peter Buttigieg, Tulsi Gabbard, Elizabeth Warren, Andrew Yang and several others are already up and running. And more are likely to follow.

The composition of the Democratic field represents the diversity of the country. With so many serious frontrunners who are not, for a change, older white men, the Democrats are showing that they are a party representing the future of the country. In contrast to the kind of rhetoric that has come from President Donald Trump, an ode to the whiteness and masculinity of a limited American political tradition, the Democrats are making a strong statement about what their party is all about.

And their opening speeches reflect that reality as well. They have revolved around issues such as racial justice, gender equity, immigration rights and more. Even a candidate like Sen. Amy Klobuchar, who is being tagged as the "moderate" with bipartisan appeal, proclaimed : "Stop the fear-mongering and stop the hate. We may come from different places. We may pray in different ways. We may look different. And love different. But all live in the same country of shared dreams."

Then comes Biden. The former vice president has every right to run and certainly has a great deal to offer. During his career in the Senate and in the White House, Biden was a strong champion of the middle class. He pushed for the kinds of policies that Trump, despite all his promises to the forgotten Americans, never pursued. As vice president, he headed the Middle Class Task Force , which championed policies such as capping student loan payments and improving retirement security. He was even ahead of President Barack Obama on the issue of same-sex marriage.

Also, as an older white male (who would be 78 years old in January 2021), there are a number of Democrats who see him as the "safe choice," the only person who will be able to withstand the President's testosterone-filled campaign. And CNN polling shows that right now most believe he would do best in a matchup against the President.

But is Biden really the best answer for the party? There is good reason for Democrats to be skeptical and to consider supporting other candidates.

Most problematic is the fact that Biden has run for the presidency several times, and each time he has struggled under the intense spotlight. When it comes time to hit the campaign trail, Biden has never been able to generate the level of support that is necessary to win. His tendency to make gaffes has also hurt him. And, in our frenzied news cycle with endless outlets and few filters on information, Biden's style will cause problems again.

JUST WATCHED Bring on Biden? Poll shows majority of Dems want the former VP to run in 2020 Replay More Videos ... MUST WATCH Bring on Biden? Poll shows majority of Dems want the former VP to run in 2020 06:58

Of course, these issues didn't stop Trump in 2016. But unlike Trump, it is unclear that without a comparable partisan media outlet (Fox News) and unyielding partisan loyalty, that Biden would survive. While MSNBC could offer a boost to Biden, social scientists have shown that the conservative media has a much stronger hold on conservative viewers than liberal outlets. Conservatives also tend to be more ideologically homogenous and loyal than their counterparts on the left. In other words, Democrats have seen the challenges that the "inevitable" frontrunner faces, as the election actually takes shape.

More important, though, is the moment in which we are living. Trump has run a political operation from the White House that has revolved around a vision of white nationalism that challenges the values of pluralism, diversity and inclusion. His statement after Charlottesville forever marked his term in office. And the President's perpetual struggle to build a wall perfectly captures what he hopes to achieve -- with his central goal being about keeping people out rather than bringing them in.

With so many strong candidates already lined up to run in the Democratic primaries, many of whom who have launched their runs with the kind of gravitas and excitement that bodes well for a presidential campaign, there is a risk of someone like Biden being in the race -- someone with the potential to suck up all the oxygen in the room. "Uncle Joe" could quickly take away media attention and campaign funds from Democrats who would perform better in the general election. If Biden was the nominee, he would also throw cold water on the excitement and energy that exists among younger Democrats about the direction of their party.

The Democrats should not assume that an older, moderate white man is inevitably the strongest candidate. Even Bernie Sanders offers a fresh perspective with his left of center, Democratic Socialist positions (he would also be notable as a Jewish-American nominee). It is possible to see the virtue of a ticket that embodies diversity from the top down. All of the candidates who are currently running would enable the Democrats to showcase the difference in what the party represents compared to the GOP. As a new article in The New York Times shows, the GOP is bleeding young voters with each passing day of the Trump presidency.

With Biden at the top of the ticket, it would be much more difficult to generate enthusiasm among African-Americans, Latinos or younger voters, for instance, who will be looking to Democrats to prove that they will offer something fundamentally different. While there are early polls showing substantial numbers of African-Americans backing Biden and Sanders, these don't measure the excitement that might be generated on the campaign trail once the other candidates are better known and once their political skills have been proven.

And Biden's pivotal role on the 1994 Violent Crime Control Act, which was foundational to the expansion of the mass incarceration system, will come back to haunt him. Like Hillary Clinton, he will have to reckon with his vote authorizing President George W. Bush to use military force in Iraq, baggage that the younger generation of Democrats won't be carrying onto the campaign trail.

Stay up to date... Sign up for our new newsletter. Join us on Twitter and Facebook

Meanwhile, many educated, suburban women are looking for a candidate who will take seriously the message of the #metoo movement. This is a social movement that has rocked American society, bringing attention to the horrendous ways in which sexism and sexual assault continue to play out in many of our major industries and in our private lives. They won't be happy to hear about his role in the 1991 Clarence Thomas hearings, where he was one of the many senators who did not take seriously the sexual harassment complaints of Anita Hill. He was part of the reason that 1992 saw a surge in female candidates.

The most successful Democrat in recent decades was Barack Obama, who mobilized a powerful electoral coalition because of the change he represented for the Democratic Party -- not because he was the party's safe choice.

Of course, Biden has a right to run -- should he so choose. But before Democrats go overboard in shifting their financial and organizational resources, as well as endorsements, to the former vice president, they should think long and hard about whether they want the face of their party to be someone who represents the past -- or the future.