WASHINGTON (BLOOMBERG) - Under attack from Mr Donald Trump and following a lacklustre fund-raising quarter, Mr Joe Biden attempted this weekend to reassure his top donors while acknowledging he was slow to respond forcefully to the United States President's accusations of corruption.

Mr Biden's campaign gathered about 100 top donors for a summit in Philadelphia, where it delivered briefings from top campaign staff, a tour of its campaign headquarters, and a 90-minute address from the former vice-president.

The campaign laid out its strategy, with under four months until the first nominating contest, the Iowa caucuses, on Feb 3.

"Going into this meeting, I was concerned," said Mr Dick Harpootlian, a South Carolina state senator who hosted a fund raiser for Mr Biden in May and attended the two-day conference. "Going out of the meeting, I am enthusiastic and optimistic."

The summit came at a critical moment for Mr Biden. His front-runner status has eroded as Senator Elizabeth Warren moved into a statistical tie with him or took a slight lead in key state and national polls.

Mr Trump and his GOP allies have unleashed unfounded corruption allegations about his son Hunter's business dealings, and Mr Biden finished the third quarter of fund raising behind Ms Warren, Mr Bernie Sanders and Mr Pete Buttigieg.

'UNPRECEDENTED' ATTACKS

Mr Harpootlian said the donors expressed their concerns about the strength of Ms Warren's candidacy and Mr Trump's increasingly aggressive attacks on Mr Biden and his family. He said Mr Biden, 76, acknowledged that he was slow to counter Mr Trump's attacks.

"He voiced a concession that this is unprecedented, and it took him a beat or two longer than perhaps it should have to respond in a very aggressive way," Mr Harpootlian said of Mr Biden.

"That speaks to a positive aspect of Joe Biden's character. You want someone to be deliberate, not shooting from the hip. Now that he's engaged in this, he says he is going to beat him like a drum."

Mr Harpootlian said the campaign was armed with a clear strategy for moving forward and many donors left feeling energised to raise more money for Mr Biden.

"In 20/20 hindsight, he should have come out swinging, two nanoseconds" after the first Trump accusation, Mr Harpootlian said.

"I think it's an affirmation of what a deliberate and decent guy Joe Biden is. Before he throws the first punch he wants to make sure that's the right thing to do. Again, this is such an unprecedented act."

BLISTERING SPEECH

Mr Biden has begun to take a much more aggressive posture towards Mr Trump. He gave a blistering speech last week in Reno, Nevada, assailing the President for abusing his power and smearing Mr Biden's family. He penned a Washington Post opinion piece last Saturday (Oct 5) assailing "conspiracy theories and smears".

Mr Biden's campaign is spending US$6 million (S$8.3 million) on broadcast and digital ads in the four early primary states, characterising the President as "unhinged", and the campaign has been aggressive in rebutting Mr Trump and his allies' attacks.

Nevertheless, the Trump campaign is also running millions of dollars on advertising that tries to link the former vice-president to Mr Hunter Biden's work as a director for Burisma Holdings, a Ukrainian energy company.

But Mr Biden's campaign has begun a rapid response arm to the fight, making it a constant back-and-forth between the two men on Twitter and elsewhere.

Mr Biden has said that Mr Trump is attacking him because he's afraid of running against him. Mr Trump tweeted on Sunday that he would "love" to run against Mr Biden, and the Biden campaign put out a statement pointing to numerous polls showing Mr Biden would beat Mr Trump in a general election.

BIG-DOLLAR FUND RAISING

According to a schedule of the donor event obtained by Bloomberg News, guests gathered at campaign headquarters last Friday afternoon for a tour and then continued to a cocktail reception at the Continental Mid-Town hotel.

Last Saturday, donors spent the day at the 201 Hotel for briefings and meetings. The campaign didn't comment on the meetings.

Mr Harpootlian said the weekend's programme was run by Mr Steve Ricchetti, who served as Mr Biden's chief of staff in the White House, and included briefings from Mr Greg Schultz, the campaign manager, Mr Pete Kavanaugh, the deputy campaign manager, and Ms Kate Bedingfield, the communications director.

The donor summit reflected Mr Biden's traditional fund-raising approach, which relies heavily on big-dollar, in-person fund-raising events. In the last week of the quarter alone, Mr Biden held 10 private fund-raising events across the country, with only two campaign events. By contrast, Ms Warren and Mr Sanders have sworn off traditional fund raisers and have found much success in a small-dollar, online approach.

In the third quarter, Mr Biden raised US$15.2 million, which put him behind Mr Sanders and Ms Warren, both of whom raised more than US$24 million. Mr Buttigieg, the openly gay mayor of South Bend, Indiana, also outraised Mr Biden, bringing in US$19.1 million. The lacklustre haul ignited new concerns about Mr Biden's durability in the tight primary with a historically large field.

But, Mr Harpootlian pushed back against fund-raising concerns and said the weekend injected new excitement into Mr Biden's top donors.

"I heard an excitement and enthusiasm from Biden that I haven't heard before," he said. "He's much more upbeat than I've seen."