Former Vice President Joseph R. Biden Jr. will hold his first event of the 2020 campaign with former President Barack Obama on Tuesday, and more than 120,000 people have already paid to attend, according to the Biden campaign, raising more than $4 million.

The joint appearance, which will be the biggest grass-roots fund-raiser of the cycle for the Democratic Party, will serve not just as a coming-out party for the former running mates but also as something of a punctuation mark on Mr. Biden’s arrival as a financial force in his own right.

In May, for the first time, Mr. Biden and the Democratic National Committee outraised President Trump and the Republican Party, $80.8 million to $74 million, and receipts are on pace to surge even higher in June. Mr. Biden’s online fund-raising so far this month has already surpassed May’s $34.4 million total, according to people familiar with the matter. Now, some party officials see $100 million as an achievable goal for June.

“May is the floor for June,” declared Tom Perez, the chairman of the D.N.C., who, along with senior campaign officials, declined to comment on the potential to reach $100 million.

The outpouring of cash has allowed Mr. Biden to sharply cut into the enormous financial advantage that Mr. Trump and the Republican National Committee built in the lead-up to 2020, shaving tens of millions of dollars off what had been a $187 million edge entering April. Since the beginning of March, Mr. Biden and the D.N.C. have banked more than $100 million.

Mr. Biden’s at times anemic fund-raising was one of his most glaring weaknesses during the primary race, when he was often badly outspent by rivals. The recent surge in donations comes as Mr. Trump appears increasingly vulnerable, reeling under the pressure of a national health crisis, an economic collapse and a wave of protests over racial injustice. Mr. Biden leads Mr. Trump in almost every national poll.

Still, Mr. Trump remains a prolific fund-raiser, reportedly raising $10 million at a recent dinner, and he has a significant cash advantage, even if it is no longer triple that of Mr. Biden.

Mr. Biden’s brightening financial picture is the result of a rapid confluence of events.

The primary race ended earlier and the Democratic Party coalesced faster behind the former vice president than expected, sparing him the expense of a drawn-out contest across dozens of states. The coronavirus pandemic sharply shrank the cost of campaigning, as Mr. Biden sheltered in place in Delaware for nearly three months. He did not need to add staff as quickly or as robustly as he otherwise might have.

At the same time, top Democratic donors have widely embraced virtual events, willingly forgoing some of the traditional perks of attending lavish in-person fund-raisers while cutting checks for up to $620,000. And as Mr. Trump falters on the national stage, small donors have seized at the chance of ousting him.

“Donald Trump is the best poster child for Democratic fund-raising in the history of Democratic politics,” said Chris Korge, the national finance chairman of the D.N.C.

Marc Nathanson, a veteran Democratic fund-raiser who helped host a Biden event on Friday, said the minimum price to get on that call was $50,000, and they doubled an initial goal of raising $1 million.

“We raised over $2 million on a Zoom call of all things,” Mr. Nathanson said.

Mr. Biden’s advisers see 2020 largely playing out as a referendum on Mr. Trump. The president’s erratic response to world events — the threats to sic the “most vicious dogs” on protesters, the forcible removal of peaceful demonstrators for his photo op outside a church, his use of racist language in calling the coronavirus the “kung flu” — has served as an accelerant for grass-roots giving, in particular as Americans took to the streets nationwide to protest systemic racism and police brutality.

About six weeks ago, the D.N.C. saw about 20 unsolicited $1,200 donations show up online — unusually large sums to arrive unexpectedly. Mr. Perez asked his team to investigate. In turns out, people had decided to essentially forward their government stimulus checks to defeat Mr. Trump.

“We actually think that we’ve become a really powerful place where people feel like they can do something about what’s happening right now,” said Jennifer O’Malley Dillon, Mr. Biden’s campaign manager.

For many months, Mr. Trump’s team has boasted about its prolific fund-raising hauls and swelling list of online supporters, with Brad Parscale, the president’s campaign manager, calling his operation a “juggernaut” in October, then again in January and February and May.

But the flip side of the enormous $817 million raised by the Trump campaign and the R.N.C. since the beginning of 2019 — and the $265 million still in the bank at the end of May — is that Mr. Trump and the Republican Party have already spent more than half a billion dollars and yet still entered the summer of 2020 trailing in the polls, with Mr. Biden cracking 50 percent in one prominent polling average. Mr. Trump spent $22.6 million on television ads from mid-March to mid-June, according to data from Advertising Analytics, a media-tracking firm; Mr. Biden just went on the air on Friday.

“The Republican war chest continues to dwarf that of Joe Biden and the Democrats,” the Trump campaign said in a statement over the weekend. (The Biden team has not released its exact cash-on-hand total, but campaign records indicate it is from $120 million to $150 million.)

Now, money is coming from all corners. The Biden campaign processed more than 900,000 online contributions in May on ActBlue, the main online portal for Democratic giving, and more than half of the donors were new to the campaign. This month began even faster, as Mr. Biden invested millions in online ads and expanded his email list by 1.5 million people, tapping into the activism arising from the protests.

Online donations were up 62 percent at the D.N.C. over the first 10 days of June compared with the same period in May. Proceeds from direct mail are booming, too: The committee saw its best May for direct mail since 2004, and the Biden campaign saw a large increase as well, according to party and campaign officials.

Overall, the number of donors to Mr. Biden has tripled since February.

“Its increasingly clear we’re going to be highly competitive with our resources against Trump,” Ms. O’Malley Dillon said.

Money alone does not decide presidential elections. If it did, Hillary Clinton would have won in 2016, and Mr. Biden would not be the presumptive Democratic nominee. But more cash gives campaigns greater strategic flexibility, allowing, for instance, Mr. Biden to buy his first flight of general election television ads last week.

Of late, though, Mr. Biden has not just raised more money than Mr. Trump — he has spent less. The Biden campaign spent half as much as Mr. Trump’s main campaign committee in May — $11.7 million compared with $24.5 million, according to Federal Election Commission records.

Mr. Trump’s campaign and the R.N.C. are paying top staff members significantly higher salaries than the Biden campaign and the Democrats. More than 20 of Mr. Trump’s campaign aides and R.N.C. officials are paid a higher salary that Mr. Biden’s campaign manager, for instance. Mr. Parscale is paid more opaquely, with monthly payments of $47,797 going to Parscale Strategy L.L.C., quadruple what Ms. O’Malley Dillon has received.

The highest paid officials were the party leaders, and there was a gap there, too: Ronna McDaniel, the R.N.C. chairwoman, was paid more than $24,000 in May; Mr. Perez was paid less than $16,000. (“A woman in the same position is making more because she is beating her male counterpart in nearly every metric,” said Michael Ahrens, an R.N.C. spokesman.)

In May, Mr. Trump spent $470,925 on polling, including $98,000 to the firm of John McLaughlin. Mr. McLaughlin wrote a memo this month titled “Skewed Media Polls,” which criticized surveys that show Mr. Trump losing the election, and the memo was recently posted on Twitter by the president. In contrast, Mr. Biden’s campaign spent only $122,300 on polling. (The R.N.C. spent another $2.5 million that was listed as “polling services/consulting” last month, which a party official said encompassed its voter data operation; the D.N.C. listed zero polling expenses.)

The Trump campaign and Republican Party also spent far more on legal fees — $1.55 million to $875,000 — than Mr. Biden and the Democrats in May.

One of the biggest shifts in the cash race is that Mr. Biden’s campaign is now regularly holding multimillion dollar fund-raisers, partly because the contribution limits for the presumptive nominee and the party are more than 200 times as high as during the primary. In June, Mr. Biden has raised a combined $21.6 million from just six of the virtual fund-raisers for large donors he has held.

And while Mr. Biden, during the primary race, had to compete with the next generation of Democratic talent, he is now able to leverage their networks, particularly those under consideration to be his running mate.

Senator Elizabeth Warren of Massachusetts held an event with Mr. Biden this month that raised $6 million in an evening, the campaign’s largest single fund-raiser. Unlike the Obama event, Ms. Warren’s relied on some major contributors. A few days earlier, Senator Kamala Harris of California organized an event that pulled in $3.5 million; Gov. Michelle Lujan Grisham of New Mexico helped host another event that an organizer said raised $1.7 million; and Susan Rice, the former United Nations ambassador, headlined a fund-raiser last week that Mr. Biden did not attend.

The biggest contributors — even those cutting checks for $100,000 or more — have been willing to bypass the traditional grip-and-grin photo lines of big donor events of the past. Some even said the virtual fund-raisers had their own charm.

“There is an enhanced intimacy with these Zoom meetings,” Sarah Morgantheau, a Biden fund-raiser, said. “They have the gallery feature and you can see everything.”

They’re also much cheaper, saving the campaign money and time.

“You don’t have to buy wine, you don’t have to rent a room, you don’t have to pay for catering,” said Michael Marquardt, another fund-raiser for Biden.

“With or without a pandemic in 2024,” he added, “I think virtual fund-raising is here to stay.”

Rachel Shorey contributed reporting.