The top fundraiser in the Democratic presidential field was hospitalized for a heart attack, the longtime polling leader and his son sit at the center of an impeachment inquiry, and the one candidate with clear momentum faces persistent doubts among some party leaders that she is too liberal to win the general election.

With breathtaking speed, the events of the past two weeks have created huge uncertainty for the candidates who have dominated the Democratic nomination race, shaking a party desperate to defeat President Trump next year and deeply fearful of any misstep that risks reelecting a president many Democrats see as dangerously unfit for office.

Concerns have risen in recent days that the potential Democratic slate has been weakened by events largely out of the candidates’ control. Sen. Bernie Sanders (I-Vt.) promised a speedy return to the campaign trail after leaving the hospital Friday, but it was unclear whether the 78-year-old would be able to replicate his previously frenetic travel schedule. Former vice president Joe Biden, who has spent most of the race as the leader in the polls, has faced daily attacks from Trump over largely unfounded allegations about his son Hunter’s foreign business dealings, highlighting a potential vulnerability for the candidate many saw as the best hope for beating Trump.

“This is crazier than most. It’s the normal twists and turns — on steroids,” said former senator Barbara Boxer (D-Calif.). “Anything can happen. Anything can happen.”

Interviews with more than two dozen Democratic leaders, top strategists and former elected officials revealed that most are still holding out judgment about how — or how much — the events will shift the race.

But they point to several worrying factors, including questions about whether Biden is equipped to mount an effective defense against Trump’s attacks and whether the surging Sen. Elizabeth Warren (D-Mass.) would alienate moderate voters and donors if she were the nominee. Some fear that Sanders’s health problems put a spotlight on the advanced age of the top contenders, all of whom are in their 70s. Others expressed skepticism that any Democrat would be able to compete against Trump’s unmatched ability to shift the public’s focus.

“We need to be proposing an alternative, and I think it is going to be very difficult to get any of that through when we are talking about impeachment,” said former senator Heidi Heitkamp (D-N.D.), who lost her reelection in 2018 in the face of high Trump popularity in her state. “ ‘My ideas are better than your ideas’ has now been eclipsed by the news of the day.”

The shifting dynamic has already altered campaign strategies, as some trailing candidates focus on early-voting states such as Iowa and New Hampshire, where they can more easily avoid the nationally broadcast din of impeachment news. But for the front-running trio, the events of the last two weeks have served as a public test of how they would handle crises in a general election.

Trump’s early and fierce focus on Biden — which includes public requests for foreign leaders to investigate his family — has challenged the former vice president to prove his ability to fight back.

“This is not about me, this is not about my son. There’s not a shred of evidence of anything done was wrong,” Biden told reporters Friday. “Let’s focus on the problem. Focus on this man. What he’s doing that no president has ever done. No president.”

But there are signs that Trump’s attacks are having an effect, even among prospective Biden supporters. One Biden volunteer, Marcie Lammers of Las Vegas, who showed up to his Las Vegas event last month wearing a Biden T-shirt, said the backlash to the impeachment news was clear in the phone calls she has been making for her candidate.

“I know it’s getting in the way,” she said of the impeachment talk and Trump’s accusations about the Bidens’ actions involving Ukraine. “When I do the phone banking, at first people said ‘Well, I’m thinking about Joe or I’m leaning toward Joe.’ Now, the people that do pick up the phone, they say, ‘Well if he wants to collect money, why doesn’t he just get it from Ukraine?’ ”

The danger for Biden, strategists say, is that voters look past the details of Trump’s charges of corruption against him, many of which have been debunked, and fear the impact. Biden sought the dismissal of a Ukrainian prosecutor who had previously investigated a company where his son worked — but he was carrying out U.S. policy with the support of other Western governments, not intervening on behalf of his family. Hunter Biden was not accused of any wrongdoing as part of investigation into his Ukrainian employer, which was dormant at the time his father sought the dismissal.

“Whether it is true or not, you have to take it seriously that it is something that might stick, even if there is no there there,” said Glen Caplin, who served as a top presidential campaign adviser for Sen. Kirsten Gillibrand (D-N.Y.), who ended her campaign in August.

“We saw it in 2016,” he added, referring to charges of corruption that Trump leveled against former secretary of state Hillary Clinton.

At the same time, several party strategists say Biden still has an opportunity to take advantage of the attention Trump has directed at him. The Trump campaign and the Republican National Committee have announced $10 million in spending on campaign ads that misleadingly attack Biden for his work in Ukraine. Biden has begun airing an ad that directly takes on Trump and accuses him of trying to engineer the selection of the Democratic nominee.

“Biden’s best positioning is when it is him versus Trump in the primary,” said David Plouffe, a top adviser to Barack Obama’s two presidential campaigns who now hosts the Campaign HQ podcast. “It’s an opportunity to show people: ‘Trump is afraid of me. I can stand up to him.’ I would seize that.”

One factor raising alarm is that Biden raised only $15.2 million over the past three months, putting him in fourth place among the candidates and well behind Sanders and Warren, who have built grass-roots fundraising networks.

“That’ll be the discussion. He’s the front-runner. Why is he not raising more?” said Terry McAuliffe, a former Virginia governor and a prolific fundraiser who ran the Democratic National Committee. “This is an opportunity with everything going on. Get yourself on the news and do it.”

The Trump onslaught has revealed growing frustration among some Biden supporters that top party officials — as well as some of his rivals — have not done more to rally behind him during what they view as a moment of crisis fomented by Trump.

“I’m disappointed in a lot of the leaders in our party who are allowing the GOP to do what they do very well,” said Harold A. Schaitberger, president of the International Association of Firefighters and a fierce Biden backer. “They have their guns blazing. They have a great coordinated plan. The basic Democratic comment is just crickets. . . . All I know is our side better learn to fight better.”

Democratic leaders and strategists also expressed concern that Sanders’s recent illness could raise fresh questions about the septuagenarian polling leaders. Warren, 70, is the youngest, and both Biden, 76, and Sanders, 78, would be the oldest president ever to take office if they won.

Sanders started the week on a high note, announcing a significant third-quarter fundraising haul of $25.3 million, larger than every other Democratic candidate. But hours later, he was hospitalized for treatment of a blocked artery. He was released Friday but is taking an unknown amount of time off. It remains unclear whether he will hold events before the next debate on Oct. 15.

“The age issue was not so much a health-related issue, but rather more one about generational appeal,” said Donna Bojarsky, a California Democratic consultant who has donated money to Democratic Sens. Kamala D. Harris (Calif.), Amy Klobuchar (Minn.) and Cory Booker (N.J.). “Now, because of Senator Sanders, maybe the issue will come up more.”

The three top-polling candidates have promised to release medical records before the Iowa caucuses, but the Sanders campaign was not fully transparent about his condition. The announcement by his doctors of the heart attack, in a campaign-issued statement, came three days after he arrived at the hospital.

The flurry of impeachment news and distractions for Sanders and Biden have accrued so far to Warren’s benefit, disrupting other Democrats’ efforts to more directly challenge her ideas and background. Warren took the lead in several state and national polls last month, and she is widely seen as building the most accomplished campaign.

“My sense is the person who is getting the best of this is Elizabeth Warren. She’s not being attacked. And she’s raising money,” said Mitch Landrieu, the former mayor of New Orleans. But any pause in the scrutiny she may face is likely to be temporary, he added.

“She has a plan for everything except for how to beat Donald Trump. That needs to get tested,” Landrieu said. “She says she can do all these things. There’s a thing called political reality. . . . Aspiration is wonderful, but you can’t eat aspiration for lunch and send your kids to college on it. That’s a fundamental decision that Democratic primary voters need to make a decision on.”

The now-overshadowed attacks focused on whether Warren was the person she claimed to be, and whether her plans, including Medicare-for-all, would alienate general-election voters.

Biden said Warren was not being candid with voters about her policy plans, and his aides had pointed to the vice president’s release of decades of tax returns, highlighting the years when Warren did corporate legal work before entering public office. South Bend, Ind., Mayor Pete Buttigieg has called Warren “extremely evasive” about the details of her health plans. Both Harris’s and Sanders’s campaign staff have pointed out that Warren, who boasts regularly of her grass-roots fundraising model, transferred $10 million into her campaign, including money she had solicited from wealthy donors during her Senate efforts.

“What has impressed me about her is she creates her own weather. She has risen in the polls on her own, not pushing off against somebody else,” said Jennifer Palmieri, a top strategist for Hillary Clinton in 2016. “I hear from a lot of campaigns that she is going to have to answer questions now. Well, I don’t know. She gets asked questions all the time. I’m not sure that confronting her is going to have the impact that other campaigns want.”

As Warren has steadily marched upward in the polls, the reality that she could become the nominee has unsettled some of the party’s top donors, who worry that she would hand the race to Trump. If it starts to look like Warren will win the party’s nomination, a longtime Democratic bundler said, “there will be efforts to stop that.”

“Right now, they think Joe Biden stands between us and Elizabeth/Bernie — and to defeat in 2020,” said the bundler, who spoke on the condition of anonymity to describe private conversations. “So if he [Biden] doesn’t measure up, that’s where you start to feel angst in the donor community.”

Another major Democratic donor, speaking on the condition of anonymity to describe private conversations, said he and other donors seeking a moderate as a nominee have “zero interest in coming around to Elizabeth Warren.” If she’s the nominee, “we can’t vote for her or Donald Trump” and would sit out the election, he said.

The donor also expressed worries that if Warren is the nominee, her presence would ruin any Democratic chances to win the Senate, because voters would perceive having a Republican majority as “the only way to keep her in check” as president.

For the rest of the Democratic field, which has been struggling in single digits or less in polls, the disruption at the top has provided a shred of hope that they might still be able to break out in one of the early-voting states.

“One or two people are going to emerge out of that second tier and be viable because I don’t think that the top tier is so strong that they can lock this down,” said Aaron Pickrell, who helped run Obama’s two winning presidential efforts in Republican-leaning Ohio. Asked about the general election, he admitted, “I’m still nervous about all of it.”

Democratic Party rules are set to narrow access to the November and December debates, possibly excluding candidates such as former housing secretary Julián Castro, Klobuchar, former congressman Beto O’Rourke (Tex.) and Rep. Tulsi Gabbard (Hawaii).

But many candidates may continue to campaign without debate access, betting on early-state voters to lift them from obscurity amid the ongoing impeachment turmoil. That is particularly true among those who see themselves as alternatives to Biden.

“There are some people who think it will fix the field where it is because it will take the oxygen away from the presidential campaign, and I doubt that is true,” said Sen. Michael F. Bennet (Colo.), who polled at zero percent in the most recent Des Moines Register poll in Iowa. “I think the big thing that is shifting is that the voters in these early states really for the first moment are beginning to pay attention to the race. Everything up to now has been preseason.”

Rep. Tim Ryan (Ohio), another struggling candidate, said he has newfound interest from donors worried about Biden’s campaign and Warren’s ideology, and recently was meeting with newly interested donors in Los Angeles. He’s also putting together meetings in New York.

“For us, I think there’s new life into the campaign because of that,” he said.

“I’ve been saying how fluid this is,” he added. “It’s just wide open, and uncertainty of impeachment adds to it.”

Annie Linskey, Michelle Ye Hee Lee, Sean Sullivan and Cleve R. Wootson Jr. contributed to this report.