DES MOINES, Iowa (AP) — A fire truck, a marching band and hundreds of supporters ushered Joe Biden into this weekend’s Iowa Steak Fry, a show of force intended to solidify the former vice president’s front-runner status. His closest rival, Elizabeth Warren, slipped in with little fanfare, delivered her speech and hit a brief selfie line before departing the show.

Their approaches to the steak fry were as divergent as their views on the role of government. But Warren and Biden increasingly face the same challenge: the pressure of being on top.

They’re entering a critical phase of the Iowa campaign in a close race for first place. For Warren, it’s a sign that the investments she’s made in staff and personal interactions with voters have paid off. But it also means she’ll increasingly be the subject of attacks from her rivals who want to blunt her rise. For Biden, months of attacks have done little to erode his standing among Democrats. But Warren’s strength underscores his weaknesses among progressive voters, ensuring he won’t be able to coast to success in Iowa or any of the other early voting states.

There’s plenty of precedent for candidates doing well in Iowa the summer before the caucuses only to fade when voting nears. With the caucuses just over four months away, more than a dozen other candidates are increasingly desperate to do whatever they can to overtake Biden and Warren. The dynamics suggest a volatile period ahead as Democrats begin to more seriously grapple with who they want to take on President Donald Trump next fall.

“Anything can happen,” said J. Ann Selzer, the longtime director of the Iowa Poll, produced by The Des Moines Register and its partners.

Selzer managed a poll released Saturday by The Des Moines Register, CNN and Mediacom, which found Warren running about even with Biden, who led their last poll in June. The survey showed more than 60% of likely Democratic caucusgoers could still change their minds on who to support.

A number of lower-tier candidates who’ve staked their candidacies on Iowa cite that large chunk of undecided voters as evidence they still have a shot, even as their campaigns stall.

Sen. Cory Booker of New Jersey, who boasts the most Iowa endorsements and a strong campaign team, warned supporters he’d have to raise big money fast or drop out. Sen. Amy Klobuchar of Minnesota has staked her campaign on the idea that her Midwestern roots would endear her to Iowans, but she’s stuck in low single digits despite frequent trips to the state. And former Texas Rep. Beto O’Rourke, who drew mobs of supporters to small Iowa towns when he launched his bid, is now campaigning beyond the early primary states in search of a win.