At a fundraiser in Los Angeles last week, Biden acknowledged how much of the debate is out of any candidate's hands.

“The deal was, well no more than 10 people on the stage for three hours,” Biden said, according to a pool report. “Well, now there’s 12. They’re supposed to, if more than 10, they’re supposed to divide it into two. At least if you had a debate with five other people you might actually get a chance to say something. But I’m going to try to be more declarative, but not argumentative.”

Biden said, “One of the problems I’m finding, I’ve got to be more aggressive."

Other presidential candidates are still grappling with what to expect from the addition of a newcomer, billionaire Tom Steyer, and a candidate who sharply criticized her fellow Democrats before missing the last debate, Rep. Tulsi Gabbard.

“Who knows what goofy bullshit Steyer will pull, or Gabbard will pull,” an adviser to one candidate said, referring to their urgent need to have a breakthrough moment.

Gabbard last week threatened to boycott the debate, while Steyer said his sole plan is to “present myself to a whole bunch of Americans who don’t know who I am, to try and explain why I’m running, why I think what I’m saying is important and what the future should look like for America.”

Sen. Bernie Sanders (left) is recovering from his heart attack and former Vice President Joe Biden is under siege, laying into President Donald Trump. | Win McNamee/Getty Images

But with Sanders' heart attack, impeachment and Trump's move to withdraw troops from Syria, it is unclear how much time any candidate will have to make such a case on their own.

“Before, they were setting their own policies and their own conversation," said Sean Bagniewski, chairman of Iowa's Polk County Democrats. "Now, they're very much all talking about the same things."

Bagniewski said the focus on impeachment and Trump’s foreign policy may help voters better compare candidates in the still-sprawling field, with external events training their attention on the same subjects.

“People are really being distinguished on the same set of issues … impeachment, what’s happening in Syria and a lot of other places,” he said. “People are actually starting to look at who would be the best leader in challenging times like these.”

For the candidates themselves, said Marc Farinella, who ran then-Sen. Barack Obama's presidential campaign in North Carolina in 2008, said, “It’s probably really frustrating to them, I would think — the fact that the latest policy announcements or what they’re doing on the campaign trail hardly gets any attention at all, even for the leading candidates.”

He said, “The impeachment stuff kind of blacks out everything else. It blacks out a discussion of policy. It blacks out anything that’s going on on the campaign trail. It’s huge.”

For months, the 2020 presidential primary had marched forward largely on its own terms — a steady beat of policy proposals, fundraising, minor skirmishes between rival candidates and organizing offensives in the early nominating states.

But unlike in previous months, few presidential observers this week are contemplating how many times Biden will associate himself with Obama in the upcoming debate, or how much time will be devoted to climate change.

Greeting reporters following an LGBTQ forum last week, Sen. Cory Booker noted the significance of the event but said — rightly — that "I suspect that that's not going to be what you want to talk about."

“Politics is unpredictable,” said Mark Longabaugh, a senior adviser to Sanders' 2016 presidential campaign. “Obviously, the Sanders team had not planned for the candidate to have a health incident, but it happened. Now you have to deal with it and move forward. I’m sure, to some degree, the Biden folks would rather not have to deal with the president’s corruptible behavior … But they’re stuck in the middle of it.”

He said, “But here it is. So, for them, it’s an external event that they’ve got to be able to roll with.”

Trump's unsubstantiated attacks on Biden do not appear to have damaged him with Democratic voters. It is possible they have helped. A Fox News poll last week found 21 percent of Democrats say Trump's claims about Biden and his son’s business dealings make them more likely to vote for Biden. Just 10 percent say they are less likely to. Thirty-one percent say they are less likely to vote for Sanders because of concerns about his health, compared to 15 percent who say they are more likely to.

Even before Sanders' heart attack and Trump's unloading on Biden on Ukraine, Elizabeth Warren’s surge in public opinion polls had become problematic for both Biden and Sanders — the RealClearPolitics polling average placed her ahead of Biden for the first time last week. And Warren is unencumbered by Sanders’ health scare or Biden’s family connections.

Longabaugh said, “To the degree that the Sanders and Biden campaigns have challenges they need to deal with — different challenges — I think Elizabeth Warren is in a commanding position here. She’s on message, she has been on the move and rising. She’s been running the best campaign of anybody in the field. And I think, when you’re in a race and a couple of the other two frontrunning candidates are facing challenges, and you’re still running in clean air, just to be crass about it, it’s advantage Warren.”

Sanders has taken steps in recent days to assure supporters of his health, telling CNN last week that he is “confident we’re going to be running a very vigorous campaign.” And Biden has forcefully pushed back on coverage of his son’s business activities in Ukraine, telling reporters to focus fully on accusations that Trump abused his power by asking a foreign leader to investigate Biden’s family. Biden called directly for Trump's impeachment last week.

Joel Payne, a Democratic strategist who worked on Hillary Clinton’s 2016 presidential campaign, said that while Trump’s accusations against Biden have been discredited, “a lot of the stuff that Biden is getting hit on, and being publicly cast as, is a lot of the same stuff Hillary was cast as — a creature of Washington, a tool of the system.”

For a progressive Democrat such as Warren or Sanders, Payne said, “it’s a perfect thing for them to attack if they wanted to. But I think they will both be loathe to do it … You don’t want to be seen as carrying Trump’s water or advancing the argument.”

Warren herself is not without liabilities — and Biden appears increasingly willing to criticize her for the cost of the progressive policies she proposes. In a veiled swipe at the senator known for her voluminous policy papers, Biden said last week, “It takes a proven ability to get things done … We’re not electing a planner.”

Doug Herman, a Democratic strategist, said Biden “made a pretty clean hit.” And he suggested that Biden, Warren and Sanders are all now vulnerable to attacks from an increasingly desperate field of lower-tier contenders.

It was in October 2007 — the same month in the election calendar — that Hillary Clinton turned in a dismal debate performance against six opponents in Philadelphia, when longshot Chris Dodd started a pile-on surrounding Clinton’s weaving around the question of whether undocumented immigrants should be granted driver’s licenses.

This year, Herman said, “Sanders has the coronary. Warren has several problems she can be hit on, and then you’ve got kind of the Chris Dodds of this year ... What’s Beto do? What’s Yang do? What do those candidates who are in the back of the pack do? How do they make themselves relevant, and at whose expense?”

Geoff Potter, who was Washington Gov. Jay Inslee’s debate director before Inslee dropped out of the presidential race in August said that earlier this year. “Everyone assumed that when we got to this point … there’d be a ton of fewer candidates on the stage.”

But llike many Democrats, Potter believes that the race is still fluid and that “the lesson, I think, is not to attempt to proverbially set your hair on fire.”

He said, “Maybe that’s the big variable we don’t know in all of this: to what degree to individual candidates feel, not that this is their last shot, but that the race is truly unsettled.”