After months — even years — of temporizing, Joe Biden finally jumped into the race for the 2020 Democratic presidential nomination. Long considered the front-runner for this cycle, the former vice president entered the race as the twentieth significant candidate — give or take a couple, depending on how you count — for the primaries, while still running ahead of the pack in polling. After chalking up an impressive haul of over $6 million in donations in the first 24 hours of his official campaign, observers could be forgiven for thinking that the nomination is the veteran Washington insider's to lose.

At least in the first few days, the polling momentum has seemed to solidify Biden's status in the field. Morning Consult's weekly temperature-taking of the Democratic field on Tuesday showed Biden jumping six points since his announcement to 36 percent, while Bernie Sanders continued a decline that started a month earlier, falling to 22 percent. A new polling result on the same day from Quinnipiac gave an even more dramatic contrast to Biden's launch. Among Democratic voters and leaners, Biden jumped up nine points compared to four weeks earlier, with Sanders sliding to third place behind a mildly ascendant Elizabeth Warren.

The rest of the field has fared less well. Even before Biden made it official, most of the other contenders, even those with national profiles, barely showed up on the radar. In the Quinnipiac poll, Kamala Harris gets 8 percent, two points below upstart South Bend mayor Pete Buttigieg, who picked up six points over the last month. Cory Booker leads the lower tier with 2 percent, followed by Amy Klobuchar at 1 percent, and Kirsten Gillibrand — from New York, the second-most populous state — doesn't register at all.

The Morning Consult interactive graph makes the point more plainly. Since January, only Sanders and Biden have remained consistently over 20 percent in its tracking polls. And in the same period, only Warren and Harris have polled above 10 percent -- briefly in both cases, and neither since mid-March. With this kind of domination, one would normally expect Biden's official entry to start clearing the field of the non-performers and close the drawbridge for further entries.

It's too early to know whether the field will start clearing. But can Biden close the drawbridge? Or will Biden's entry signal that the field is not complete yet? Stacey Abrams might be the first test — and perhaps not the last.

At one point, the Biden campaign was reportedly strategizing their launch in partnership with the former Georgia legislative leader as Biden's running mate. Allies of Abrams, who narrowly lost a contested gubernatorial race last year, quickly made it clear that they considered the idea “exploitative.” Abrams herself commented at the same time that the Democratic nominee would likely be a woman, a person of color, or both.

Fast forward to Tuesday, and it seems Abrams has herself in mind for that position, even with Biden in the race. Abrams announced that she would take a pass on running for the U.S. Senate in Georgia, despite Chuck Schumer's personal recruitment efforts, on the grounds that it would not be “the best role for me in this battle for our nation's future.” Perhaps Abrams wants to focus on activism, but the statement sounded more like a warning that she had set her sights higher than the Senate despite Biden's addition to a very crowded field.

Abrams has shrewdly used the notoriety over the disputes in the Georgia gubernatorial election to raise her national profile. Still, for now she has only risen to the level of leadership in a state legislature, a relatively obscure position in a race that features a half-dozen senators, several House members, a couple of governors, and now a former vice president. But Abrams isn't the only possible late entrant to the field.

What will happen if potential candidates with higher national profiles jump into the race? Former New York City mayor and multi-billionaire Michael Bloomberg has acknowledged that he's been thinking about jumping into the race. Attempts have been made to draft celebrity candidates such as Oprah Winfrey or Mark Cuban to combat Trump's celebrity and populist standing.

All three have the time, and more importantly the money, to start off with instant credibility and organization. All three have been public in their opposition to Trump, and all three have identity claims that reflect Democratic contemporary sensibilities better than Biden does. Nothing about Biden's track record as a candidate or as a fundraiser would necessarily discourage those entries. In fact, his well-known shortcomings on both counts might prompt significant concerns about allowing the nomination to default once again to a two-time also-ran.

That brings us to Biden's biggest vulnerability in the primaries, and perhaps the biggest reason to keep that drawbridge down. Biden is a candidate who was best suited to run for the nomination not in 2020 but in 2016, when he could have run as someone to safeguard Obama's legacy. He has been out of office for four years, after working in Washington continuously for over 40 years. His candidacy will have to rely on looking back in time when most Democratic voters want to look forward. Biden is an establishmentarian in a populist wave fueled even more by resentment over the right-leaning populism behind Donald Trump's ascendancy. He is, to put it bluntly, an old white male politician in a party that puts identity politics at the core of its message — a defect that Abrams saw clearly enough when Team Biden was sending up its trial balloons last month.

The longer Biden stays a front-runner before the debates, the more it incentivizes a challenge from one or more big-name candidates who speak more clearly to the Democrats' brand of today. Democrats got caught napping four years ago in defaulting to an old name in party politics. Their patience this time around might be much shorter — and their desire for fresh faces may not yet be sated.