Seattle, Washington (CNN) As the summer slog gives way to an expected fall frenzy, the Democratic presidential primary campaign is beginning to shape up as a three-way contest.

Even before Monday's Monmouth University poll put a jolt into the field, the troika of former Vice President Joe Biden Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders had been showing signs of breaking away from the historically crowded field.

Warren and Sanders, the party's de facto progressive leaders, are increasingly showing signs they are primed to run down Biden, who has maintained his polling advantage through the first few months of his campaign. The Monmouth survey put all three within a single percentage point, well within the margin of error. Still, it is the first national poll that showed anyone other than Biden in front. Supporters of Warren and Sanders can point to that and the trendlines, which see them gaining, while Biden's pollster, armed with other recent surveys, argued on Monday that his rivals are celebrating a false dawn.

Warren's climb has been slow and steady, her numbers consistently ticking up as she begins to accrue a priceless cultural capital -- see the "selfie lines," read the "plans" -- among Democratic voters. Sanders, too, is busting early perceptions about his second presidential run, showing himself to be more resilient than many expected while gaining support among minority voters, the key constituency that largely rejected him in 2016.

Biden's trajectory is more difficult to read. He enjoyed comfortable leads in other recent polls from CNN -- on the rise and nearly double of both Sanders and Warren, at 29% -- and Fox News , which had him with a similar advantage. But his uneven performance on the stump has been drawing the wrong kind of headlines, potentially undermining the former vice president's core argument: electability.

Differences on the trail

The three candidates might be drawing closer together in the polls, but the diverging paths of their campaigns were on display this past weekend, as Warren was swept up by big crowds out west, Sanders kicked up a storm in Kentucky and Biden, in early-voting New Hampshire, occasionally slipped up when he went off-script.

Seattle on Sunday embraced the emboldened Warren, with an estimated 15,000 people turning out for massive town hall event in a grassy park underneath the Space Needle. Asked by someone in the crowd how she would take on a "bottom feeder" like Trump without losing herself in the fight, Warren flashed her sharp elbows.

"You don't back down from a bully. You have to be willing to lay into this. And nobody is getting behind me on a debate stage and doing a handsy thing -- that's not happening," she said, a reference to Trump's stalking of Hillary Clinton during one of their 2016 debates.

Warren's core message hasn't changed since the early days of the campaign, but her audiences are getting larger as liberal affection for both her policies and political style grows alongside them.

"We're not going to win this by just saying 'not Trump.' We're not," she said in response to the question about the President. "We should call him out on specific policies, we should make sure people understand where he stands, when he's ugly, we should call it out, but that's not enough. It's not enough to be 'not Trump.' A country that elects Donald Trump is a country in serious trouble."

Dawn Jorgenson and her 21-year-old daughter, Sophia, who said they were fans of Warren since her 2017 spat with Senate Majority Leader Mitch McConnell — an exchange that ended with an enduring gift, his frustrated observation that, "Nevertheless, she persisted" — both stood in a photo line for hours to get their "selfie" with the senator.

"I can appreciate the fact that she's standing out here spending all this time when she's probably exhausted, to meet and shaking hands. To me, that says a lot about her character and her person just to do that," Jorgenson told CNN.

Her daughter interjected while laughing, "We are hoping she gets a dinner break sometime in there." That meal wouldn't come for a while -- Warren spent four hours posing for pictures before calling it a night.

Back east, Sanders was talking about McConnell, too. At a Sunday rally in Louisville, Kentucky, he addressed the Republican leader directly. It's a tactic Sanders' campaign has leaned into over the summer as it moved away from more traditional speeches and toward more creative, confrontational spectacles, pitting the Vermont independent against powerful political figures and corporate interests.

Accusing McConnell of "cowardice" and labeling him "an obstructionist," Sanders cast the Senate majority leader as a tool of big money.

"I say to Sen. McConnell, have the courage to stand with the working people of Kentucky and America," Sanders said. "Not your wealthy campaign contributors."

A few hours earlier, Sanders -- in a last-minute addition to his schedule -- had stood on a flatbed in a parking lot, bullhorn in hand, addressing striking members of the Communications Works of America, as the union ramped up its contract stand-off with AT&T.

A day later, he was in Pittsburgh collecting his first national labor endorsement of the 2020 campaign. It came from the United Electrical, Radio and Machine Workers of America. His hands clasped and raised with union members, Sanders offered his gratitude -- and a promise.

"Brothers and sisters, this means a lot to me. I want to thank you very much. We have an enormous struggle in front of us, but I am confident that with you at my side, we're gonna win that struggle," he said.

While Sanders and Warren have campaign relentlessly through these final days of August, Biden's appearances on the trail have been more measured. His words, like his politics, are more cautious than the progressive pair, and in New Hampshire during a two-day visit, he hosted a town hall on health care while seeking out the folksy connections that have been the hallmark of his political career.

He also pulled some punches when it came to Trump, a nod to the fraying tradition of bipartisan solidarity with the commander-in-chief during diplomatic trips.

Trump's foreign policy "has me greatly concerned, but I'm not going to take the time today," Biden said at a campaign stop. "It's an omission by intention because I don't think it's appropriate for me to be criticizing an American president when they're abroad and engaging in foreign policy."

Biden and Sanders represent the ideological poles of the Democratic Party in this election cycle, but there is at least one similar question facing their candidacies: age. Biden fueled those concerns again over the weekend, referring to the G7 summit as the G8 and, when asked what he thought about Keene, New Hampshire, replying, "What's not to love about Vermont in terms of the beauty of it? I mean this is sort of a scenic, beautiful town."

When the question of voter anxiety over his age came up, Biden, 76, replied, "I say if they're concerned, don't vote for me."

At his own rallies, at least, there weren't signs of such apprehensions.

"He's been doing this kind of stuff all his life. It's not an old age thing," said Joe Cullum, who popped over from Spofford, New Hampshire, to see Biden. "And who doesn't (slip up speaking)? When you're out there doing that, it's not easy."

When a similar question was put to Sanders days earlier at the Minnesota State Fair, the 77-year-old -- who keeps a hustling campaign schedule -- issued a challenge of his own: "Follow me on the campaign trail," he said, eventually asking that he be judged "on the number of picket lines that I've been on with working people throughout my entire life."

After closing his remarks and wending his way through the fairgrounds, Sanders wrapped his appearance with a campaign custom of sorts -- a few rounds of arcade basketball.