For months, Democrats have been conducting a kind of fantasy race for the presidency. Candidates have hired staff, launched websites and posted fundraising videos, sat for media profiles, issued policy proposals, and of course visited fairs in Iowa and diners in New Hampshire, all on the premise that the field was wide open, with plenty of talent to choose from, and why shouldn't each of them be one of the choices?

Now, the fantasy phase is over, and the real campaign begins. Vice President Joe Biden has finally entered the race, and in entering it he has defined its true shape and purpose. He is the 800-pound gorilla, and every other campaign must now figure out how to out-wrestle him.

That this is Biden's race to lose should be crystal clear, even this far out. Start with the polling. Three new national polls, from CNN, Quinnipiac and Morning Consult, show Biden to be the clear front-runner, with 39 percent, 38 percent, and 36 percent support respectively, far ahead of Senator Bernie Sanders, who polled at 15 percent, 11 percent and 22 percent respectively. That's not a close contest. Historically, candidates who polled this well early in the campaign most often became their party's nominees.

Biden's strong position is also reflected in polls of early voting states. A poll of New Hampshire voters from the Boston Globe shows Biden the clear leader, with 20 percent support to 12 percent for Sanders — who won the state decisively in 2016. (Sanders was tied for second with Mayor Pete Buttigieg.) No new polls have come out of Iowa yet, but Biden was leading or tied before his announcement, and should be even further ahead today.

Biden's fundraising hasn't been extraordinary relative to his competition — his first day haul was in the same ballpark as Sanders' and Beto O'Rourke's — but it has certainly been sufficient to the moment. He has also instantly become the clear leader in endorsements, and is the only major candidate other than Sanders to have garnered significant endorsements outside of their home state. Most important, Biden's appeal — unlike Jeb Bush's in 2016 — is broad-based, encompassing both midwestern Obama-Trump voters and southern African-Americans. And he's in a very good position to pick up additional support if so-far marginal candidates who lack extensive small-donor funding bases — like Booker, Gillibrand, Castro and Klobuchar — see their support dry up in favor of the front-runner.

There are a few additional details to be gleaned from the recent polls. Sanders has clearly slipped, and is now polling much more like a member of the pack than a potential front-runner. It's possible that with Biden in the race, some soft Sanders supporters who responded to his persona and familiarity rather than his ideology have jumped ship. Elizabeth Warren, by contrast, has continued to edge upward, and is the only major candidate not to have seen her standing slip in the wake of Biden's entry. She even surpassed Sanders to take second place in the Quinnipiac national poll, though she continues to poll surprisingly poorly in New Hampshire. Still, her support is probably more solid at this point than any other candidate in the pack, given that it was built in the teeth of negative stories and media enthusiasm for other rising candidates like Beto O’Rourke and Pete Buttigieg.

As for the rest of the pack, Senator Kamala Harris' camp has the most reason to be worried, both about her recent performance and about her prospects. Harris was always going to position herself as a safe establishment choice who could appeal to both progressives and moderates and rebuild the Obama coalition. But the beloved and experienced vice president now occupies that space, and she is unlikely to pull support away from him merely by being younger, female and non-white. That is the challenge now, for all the other would-be nominees: to define their candidacies in a race against Biden.

Biden isn't Hillary Clinton circa 2016, who had the overwhelming support of the party and expected a coronation rather than a contest. He's the front-runner, but he will have to run a race and win. And Biden has a long history of self-sabotage, so for those who can afford to stay in, there's plenty of reason to do so. But Biden is probably in a position close to that of Walter Mondale in 1984 or Bob Dole in 1996. If he doesn’t beat himself, he’ll be hard to beat. Anyone aiming to try will need to do more than just present a fresh face. They'll need to make an argument.

That argument needs to be about the future, not the past, not about Biden's voting record or personal behavior, his age, race, or gender. Biden is already very well-known, and he is very well-liked. What he lacks is a persuasive case for where he wants to take the country — something that Mondale and Dole also lacked, and for which they suffered in the general election. To beat Biden, a challenger will need to make an argument about where the country has gone wrong that goes deeper than an attack on the Trump administration, and present a roadmap for response that Biden cannot easily coopt.

That's something Sanders and Warren are each prepared to do, because they're already doing it. We'll see how many others currently in the race are prepared to do the same.