In his first days as an official candidate, former vice president Joe Biden has opened a significant lead in national polls, posted the top one-day fundraising total and showcased his ability to rattle President Trump.

His surprisingly strong debut has set off alarms in opposing camps, prompting his rivals to recalibrate their strategies for the next phase of the primary fight.

Sen. Bernie Sanders (I-Vt.) has taken the most dramatic action, making a personal decision to contrast his policy record with Biden’s. Sanders’s advisers said he plans to continue that thrust, and his campaign manager is calling out candidates standing on the sidelines.

Sen. Elizabeth Warren (D-Mass.) raised money off Biden’s entrance by whacking him for soliciting checks from wealthy benefactors and separately noted under questioning that he sided with credit card companies in a key legislative battle.

Former vice president Joe Biden announced his bid for president on April 25. Here's a look at the two other times he ran for president. (Monica Akhtar/The Washington Post)

Sen. Kamala D. Harris (D-Calif.) seized on Trump calling her “nasty” by turning it into a rallying cry in social media ads that sought to demonstrate that Biden is not the only candidate who can provoke the president.

“He’s had a gravitational effect on the other candidates,” said James Carville, a longtime Democratic strategist who worked on Bill Clinton’s 1992 campaign.

Biden has benefited from the dynamic of the 2020 primary season: Democrats have put forth the most diverse slate of candidates in history, generating excitement across the party, as measured by crowds massing at their events and donations flowing to their campaigns. But no standout has emerged with staying power, creating the vacuum into which Biden, who is well known and attached to the last Democrat to win the White House, has slipped.

Sanders has proved he still has a loyal following from 2016, but he has struggled to expand his base. Harris drew 20,000 people to her launch but has been unable to maintain lasting momentum before her well-received, televised questioning of Attorney General William P. Barr. Warren’s suite of detailed policy proposals has impressed activists, but it so far has not translated to a big bump in the polls. Former Texas representative Beto O’Rourke raised heaps of cash on his first day but has yet to revive the viral excitement created in his 2018 Senate run. Mayor Pete Buttigieg of South Bend, Ind., has risen from obscurity but is untested and has yet to expand his campaign deeply in early states. All of the candidates are looking to the debates that begin in June to offer what they hope will be a breakout moment.

It is not yet clear whether Biden himself will be able to maintain his tentative hold on the race; statewide polls in early states show him in a weaker position than national surveys, and his first events demonstrated his limitations as a candidate. His speeches were often meandering and his aides sharply limited access to him — he took no questions from voters — a style of campaigning that can backfire in states where people are accustomed to taking the measure of their options up close.

“People know him and there’s a comfort level with him,” said Rob Hogg, an Iowa state senator. “But I don’t think it’s a done deal for Joe Biden.” He added, “There’s a lot of interest in somebody new, in the next generation.”

The candidates fresh to the national stage have been blunted to some extent by the presence of Sanders, the second-place finisher to Hillary Clinton in the 2016 Democratic contest and, like Biden, a septuagenarian. For reasons both strategic and ideological, he has become Biden’s sharpest critic.

Sanders jumped at the chance in recent days to compare himself with Biden on far-reaching free trade agreements and the Iraq War — which he opposed and Biden supported. The strategy is similar to the approach he took against Clinton in 2016, when he mercilessly pounded the establishment front-runner on their policy differences and exposed the leftward turn of many Democratic voters.

Sanders’s advisers say he is just getting started.

“Senator Sanders has had a lifetime of consistency around the issues that he’s raising,” said Sanders campaign manager Faiz Shakir. “And quite frankly, on many of those issues . . . Biden has been wrong on the first instance.”

When it comes to the rest of the field, Shakir said, “I’m not sure many of them are all that different” from Biden. He added, “If you’re not interested in drawing the contrast, right, it certainly makes it less clear to us that there is any distinction.”

The Biden-Sanders split embodies a broader Democratic divide. While some believe the path back to power lies in the political revolution Sanders is urging, others feel a better bet for defeating Trump is Biden’s pitch for a restoration of more conventional Obama-era politics.

Biden and Sanders represent the same side of another Democratic divide — both are running against a crop of younger candidates who are newer to elective office and whose racial and gender diversity better reflects the changing country. Yet despite coming from different ideological tracks, the two are competing for some of the same voters — white, working-class people in upper Midwestern states Trump won.

After an impressive start of his own, Sanders has dipped a bit in public polls. His crowds have diminished in recent weeks. He’s had some trouble attracting nonwhite voters. And a sizable chunk of the Democratic Party does not like him or doubts he would beat Trump.

“He’s an old, angry guy running against Donald Trump, who’s an old, angry guy,” said Barack Obama’s 2012 campaign manager, Jim Messina. “That’s not a contrast.”

The added pressure of having Biden in the race was apparent at a rally Sanders held at Iowa State University on Saturday. Ron Craig, 62, an undecided voter there, said he was leaning toward Biden.

“He might be able to get more of the swing voters, you know, that might be leery of voting for somebody who’s really far left,” he said. Craig’s main goal? “To beat Trump.”

All of the candidates besides Sanders are taking a lower profile in the post-Biden period, wagering that if he falters they will be well-positioned to inherit voters up for grabs.

Sanders’s allies are watching Warren, whose similar platform makes her a competitor for the mantle of a more liberal alternative to Biden.

Pressed by a reporter after Biden’s entrance whether he was “too cozy” with Wall Street to regulate it as president, Warren said she had defended struggling families in past battles over bankruptcy matters, whereas “Joe Biden was on the side of credit card companies.”

Since then, however, she has been judicious about taking him on.

Asked about Biden in a brief interview, Warren declined to speak about him or his record. “I can’t speak to anyone else’s campaign,” she said.

Warren is focused on outlining policy proposals; her mantra is “I have a plan” and T-shirts with the phrase have become her campaign’s fastest-selling new item.

Part of what appears to be propelling Biden in his campaign’s early days is his strength among different sets of voters, including not only white, blue-collar voters but also African Americans. Multiple candidates are also competing for that support.

Harris, who is making a vigorous push to win black voters, will address the Detroit chapter of the NAACP on Sunday.

“I adore Joe Biden,” Harris said when he joined the race.

Buttigieg began the past week by lunching with the Rev. Al Sharpton and ended it on the cover of Time magazine with his husband, Chasten.

Buttigieg’s campaign believes it needs to establish deeper relationships — and policy credentials — with voters who know little about the South Bend mayor. His team is also working to scale up its presence in early states including South Carolina, where he is campaigning Sunday and Monday, immediately after Biden’s own visit there.

While the other candidates for the Democratic nomination have taken on Biden in differing measures, the former vice president has focused solely on a contrast with Trump. He announced his run in a video highlighting the president’s remarks about a white supremacist rally in Charlottesville, prompting Trump to rehash his comments.

“I understand the president’s been tweeting a lot about me this morning. I wonder why the hell he’s doing that!” Biden said on recent swing through Iowa, practically giddy. “I’m going to be the object of his attention for a while, folks.”

He has also worked to appear in step with the current electorate. On Wednesday night, during an event in Des Moines, a half dozen protesters in penguin masks raised signs that read, “Climate is a crisis.”

“Don’t worry, I’ll get to climate change, I promise,” he said. “And by the way, I got there before any of the other candidates did, I might add.” Perhaps unintentionally dating himself, he noted, “I’m one of the first guys to introduce a climate change bill, way, way back in ’87, okay?”

Biden is also seeking to expand his financial advantage over many in the field. While some of his opponents have sworn off wooing big donors amid rising Democratic concerns about the influence of the wealthy, Biden is scheduled to appear at a fundraiser in Los Angeles on Wednesday where donations range from $2,800 to $10,000, according to the invitation.

He also is delivering constant reminders of perhaps his biggest selling point: his connection to the 44th president, who remains popular among many Democratic voters.

“I think there is a lot of excitement about him simply because he has served under President Obama,” said Rep. Lucy McBath (D-Ga.), who represents a swing district in the suburbs of Atlanta and has not made an endorsement. “People kind of believe, you know, he’s probably one of the more experienced presidential candidates.”

That sentiment so far is echoed by many voters. While they acknowledge he is not a perfect candidate, voters say he seems authentic and represents what they crave: a return to normalcy.

“As soon as he announced, I thought: Yes. Someone is coming to our rescue,” said Hope Phillips, a 52-year-old financial industry worker from Des Moines.

Andrew Lietzow, a 67-year-old from Des Moines who is executive director of the Iowa Landlord Association, is the kind of voter Biden’s rivals need to worry about. If Biden weren’t in the race, Lietzow might be supporting one of them.

“Cory Booker is strong. Elizabeth Warren is strong. So is Kamala Harris. But compared to Joe? Not even in the hunt,” he said.

Annie Linskey, Chelsea Janes, Amy B Wang and Cleve R. Wootson Jr. contributed to this report.