This week’s first Democratic debate is a test for every candidate running for president in 2020. It is the first time a national audience will have the chance to measure these candidates as the next president of the United States. And as the old saying goes, you get one chance to make a good first impression. This is it.

Already, the Democratic field has two distinct tiers. There is a top tier and everyone else. The top tier is former Vice President Joe Biden, Senator Bernie Sanders, I-Vt., Senator Elizabeth Warren, D-Mass., South Bend, Ind., Mayor Pete Buttigieg, and Senator Kamala Harris, D-Calif. The stakes are clear: Biden and Sanders have the most to lose; Warren, Buttigieg, and Harris the most to gain.

BOOKER SAYS BIDEN'S COMMENTS ON WORKING WITH SEGREGATIONISTS WERE 'HURTFUL' TO MANY AFRICAN-AMERICANS

The challenge for Biden and Sanders is to stop their slide and not lose more ground in the polls. For Warren, Buttigieg, and Harris, the goal is to improve their standing at the expense of Biden and Sanders. For everyone else, that night is a fine line between standing out and a stunt to get some attention and a second look.

Four of the five top tier candidates will debate on Thursday night. Warren is the lone top tier candidate that will appear on Wednesday night. A debate with all the top tier candidates on one night would be preferable, but Warren can still make gains without being on the same stage.

As the candidate with the most momentum going into the debate, she is gaining in the polls and on Sanders and Biden and is now tied for second place. The strategy Warren has used throughout this race is to be the first. She was the first to announce, the first with a position, the first with a plan, the first with a grassroots organization, and the first to forgo big fundraisers. As a result she has defined the race and every other candidate has reacted to her.

Biden’s effort to clarify his comments in an interview at the South Carolina Democratic Convention on Saturday made it worse, not better. Yet another failed clean-up on Aisle Biden.

Warren can be first again, defining the debate not only on Wednesday night, but Thursday as well by challenging Biden and Sanders even without either one of them on the stage. It would force both to respond to her for 24 hours, and likely during the Thursday debate as well. Warren can challenge Biden about his comments on working with segregationists to get things done in the Senate, his reversal on the Hyde Amendment, the bankruptcy fight that started between them in 2005, eliminating the filibuster to get things through an obstructionist Republican Senate, or any of her plans.

As for Sanders, he attacked Warren last week, calling her a corporate Democrat as she begins to pass him in the polls. He then proceeded to give a speech about Democratic socialism to secure his base. Warren has ample ammunition to use against Sanders, including the fact she is a capitalist and he is a socialist. Warren can also draw a contrast between her ability to get things done compared to Sanders’ woeful record in Congress, passing only 1.8 percent of the legislation he has introduced in 30 years. The last point is important to Democrats who not only want a candidate to beat Trump but also someone who can fix what he has broken.

As the front runner, Biden will be the target for most candidates on both nights, but especially Thursday night. While the biggest threat to Biden is Biden, for those on the stage with him the debate is an opportunity to try to show they are his equal.

Whether Biden responds to any challenges from his opponents is something to watch. With the exception of asking Senator Cory Booker, D-N.J., to apologize, the former vice president hasn’t engaged his competitors, or even Democrats, as much as he has run a general election campaign against Trump. That is a strategy fraught with peril, especially Thursday night. It is one thing to stay above the fray and target Trump. But, it is quite another when there are a lot of unanswered questions about his record, not to mention his latest gaffe, that a national audience will want to hear the answers.

Biden’s controversial comments last week about his willingness to work with segregationists in the past has brought the issue of race front and center to the primary campaign, and that will likely occupy some part of the debates. Biden’s effort to clarify his comments in an interview at the South Carolina Democratic Convention on Saturday made it worse, not better. Yet another failed clean-up on Aisle Biden.

The fact is African American voters don’t want Biden talking about race the way he did last week. Biden doubled down on his comments, taking a page out of the Trump playbook by refusing to apologize – and then turning around and calling on Booker to do so. The problem is, Biden isn’t Trump but he is a Democrat. Democratic primary voters don’t want someone who uses Trump’s tactics, especially about race, given Trump’s use of it to play divide and conquer politics.

Booker, who called on Biden to apologize, is likely to raise this issue Wednesday when he is on the debate stage. But, it is Kamala Harris, who will be on the stage Thursday night with Biden, who has the real opportunity to challenge him about it. Harris hinted last week that she may very well do that. And if she does, and if she brings the prosecutorial skills for which she is known, then she may well catapult herself further into the top tier of the race and knock Biden down in the process.

The Biden controversy also exposed a generational split, with younger African Americans like Ta-Nehisi Coates and Jamelle Bouie denouncing him, while older African Americans like Congressman John Lewis, D-Ga., and Congressman James Clyburn, D-S.C., are defending him. That dynamic is important in the primaries, especially in the early contest of South Carolina, and could be an issue in the general election if Biden is the nominee.

To those who seek one shining moment, be advised they fail more often than not.

The generational divide will also be on display Thursday night with 70something Biden and Sanders on the stage surrounded by candidates in their 50’s, 40’s and even 30’s. The opportunity to exploit the generational divide is greatest for Pete Buttigieg, the little-known 37-year-old Indiana mayor who has cracked the top tier over better-known and more experienced competitors. Whether it is an over-correction to Trump or over-achieving, Buttigieg has the chance to present a sharp generational contrast on the national stage. Recently, he’s been test driving a message saying that America can’t go back to the past, as in the 1990s, that is a direct rebuke of Biden. You can expect to see Buttigieg use that tactic Thursday night.

Finally, there will no doubt be an ESPN-like compilation of attempts by the other candidates in the race to get attention. To those who seek one shining moment, be advised they fail more often than not. Again, it is a fine line between standing out and a stunt. If you’re serious, then the best bet is to be the best version of yourself to get a second look. Anything more will insure you will be an asterisk in presidential history...or on YouTube.

CLICK HERE TO GET THE FOX NEWS APP

Don’t underestimate the importance of these debates. These two nights in June will have defining moments, and collateral damage, that will follow the candidates into the fields of Iowa, the snow of New Hampshire, the low country of South Carolina, and the lights of Las Vegas.

That is why the debates this week really matter – and why Donald Trump will no doubt be watching, and tweeting, too.

CLICK HERE FOR MORE FROM MARY ANNE MARSH