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. Some candidates are rethinking their Iowa strategies to better position themselves for a strong caucus showing. Sen. Bernie Sanders of Vermont, who once seemed like Biden's biggest foe, parted ways with his Iowa political director in recent weeks. California Sen. Kamala Harris seemed mildly chagrined to have to dig out her cold-weather gear as she committed, jokingly, to 'moving to Iowa' in order to resuscitate a campaign stuck squarely in the middle of the pack. Harris has pledged to double her staff in the state and campaign in Iowa every week in October. And South Bend, Indiana, Mayor Pete Buttigieg has been investing heavily in his Iowa operation and will, by the end of this month, nearly match Biden in staff numbers and field office openings. Not only did he have more than 1,000 supporters at the event, he invited more than two dozen donors in hopes of demonstrating he can win in a class of rivals with much higher national profiles. Buttigieg advisers met in Des Moines with donors Saturday evening to discuss his recent organizational surge to increase staff to 100 paid workers and 20 offices statewide, putting him on par with Warren. 'You need to be in the hunt. You need to have the resources to compete,' said Buttigieg, who raised a record $25 million in the second quarter and is expected to be competitive in the third quarter, which comes to a close next week. Biden's allies remain adamant that what Democratic primary voters most want is someone who can beat Trump, and that Biden is seen as the safest bet to do so. But some voters say as Biden has faced scrutiny for his past policy positions — and racked up repeated gaffes — that veneer of electability is showing cracks. 'I just wonder if he has what it takes anymore,' said 50-year-old Frank Hansen of Des Moines. 'I think it has to be someone younger and ready to take on the future.' Hansen's wife, Holly, 51, attended this weekend's Steak Fry. Echoing her husband, she said, 'Joe Biden was the person seen as the strongest one. I'm not so sure now.' The couple said they were looking to a newer class of candidates, including Buttigieg, O'Rourke and Warren, who, despite being 70, 'just seems new and fresh,' Holly said. Iowa political history holds warnings for Warren as well, after previous candidates peaked too early and weren't able to sustain their momentum through caucus night. In the late summer of 2003, Howard Dean was the Iowa frontrunner; he eventually fell to John Kerry after being tagged as angry and unprepared for a national race. In the fall of 2007, Hillary Clinton still led in the polls, but eventually faced an upset to then-Sen. Barack Obama after he criticized her as running a too-careful campaign. Indeed, this week Warren's opponents stepped up their attacks against her, with Biden knocking her on raising taxes to pay for a single-payer health care system, while Buttigieg called her 'evasive' about the tax issue. Warren has, however, stayed staunchly on-message, insisting when pressed by reporters this week in Iowa only that 'costs will go down' for the middle class if her health care plan is passed. Her response has been much like the campaign she's run in Iowa — steady and focused, with little fanfare and an eye on the end goal rather than the day-to-day fluctuations in the field. She shrugged off questions about her strength in Iowa on Sunday. 'I don't do polls,' she said as she joined a protest with striking United Auto Workers members. 'We are still months away from the Iowa caucuses and the first primary elections.' ___ Associated Press writer Corey Williams in Detroit contributed to this report.