James Simons, a Long Island investor and philanthropist, has not given a cent to President Barack Obama’s re-election campaign this year.

But Simons has given at least $2 million to Priorities USA Action, the super PAC aiding Obama, and another $2 million to two allied groups supporting Democrats in Congress, making him the biggest Democratic super PAC donor in the country.

With the election just weeks away — and millions of dollars in advertising time booked but not yet paid for — Democratic super PACs are finally drawing the kind of wealthy donors who have already made Republican outside groups a pivotal force in the 2012 campaign.

More than 40 individuals and couples have given at least $250,000 to the leading Democratic super PACs through the beginning of September, according to a analysis of campaign finance records, and dozens more have given $100,000 or more.

But the money is not coming from where the groups once expected it to. Few of the wealthiest men and women closest to Obama have donated, even as the super PAC backing Mitt Romney raises millions of dollars from his friends and former colleagues.

Only a few gay donors are among the biggest givers, despite Obama’s embrace of same-sex marriage last spring. Most of the wealthy liberals who financed the party’s last major outside spending effort, in 2004, remain on the super PAC sidelines.

In their place, the Democratic groups are raising heavily from the party’s traditional, pre-Obama sources of campaign cash: trial lawyers, unions and Hollywood. And at a time when Obama’s own big donors often complain about his indifference and inattention to them, Priorities USA has had more luck outside the president’s inner circle than inside it.

Simons is more typical of the emerging Democratic superdonors giving to super PACs, which can accept and spend unlimited contributions in the wake of the Supreme Court’s Citizens United decision and other court and regulatory rulings. A mathematician who founded one of the world’s most successful hedge funds, Simons is politically tied most closely to Senate Democrats, and until recently he was better known for his donations to higher education, including a $150 million gift last year to the State University of New York at Stony Brook.

Now he is a volunteer fundraiser for Priorities USA and other Democratic super PACs, and he hosted an event in Charlotte, N.C., for prospective donors during the Democratic convention.

In an email, Simons declined to be interviewed about his role.

"The fact is that I am not seeking any publicity in this matter," Simons said. "The donations can speak for themselves."

Another emerging donor is Peter G. Angelos, a Baltimore trial lawyer and majority owner of the Baltimore Orioles who has given more than $1.2 million to Priorities USA and other super PACs.

"I’ve never met the president," Angelos said. "What’s driving me is the need to re-elect the president and a Democratic Congress. Otherwise, politically, we’re going nowhere."

The super PAC donor universe could expand rapidly in the coming weeks. Mayor Rahm Emanuel of Chicago, who was tapped to help the groups but was quickly sidelined by the Chicago teacher’s strike, has resumed raising money for Priorities USA, a spokesman said.

And Thursday, Bill Clinton will headline a lunch in New York to benefit Democratic groups. His audience will be members of the Democracy Alliance, a consortium of liberal donors, many of whom have been reluctant to make large contributions to super PACs this year.

The timing could be critical for Obama. Conservative super PACs and other outside groups are preparing a coordinated barrage of negative advertising for coming weeks, seeking to reverse Romney’s recent slide in the polls. And Priorities has reserved significant amounts of airtime in advance, hoping to raise enough cash in the coming weeks to pay for it.

John Eddie Williams Jr., who made a fortune representing Texas in the 1990s tobacco litigation, has given close to half a million dollars. Labor unions have given more than $14 million to the Democratic groups, with the Service Employees International Union and the air traffic controllers’ and the pipefitters’ unions each contributing at least $1 million. Actor Morgan Freeman, producer Jeffrey Katzenberg and Haim Saban, the Los Angeles media investor, have all given $1 million or more.

Unlike Obama’s campaign, the super PAC supporting him is accepting money from lobbyists and political action committees, as are the two congressional super PACs. Perennial Strategy Group, which has represented commercial banks and home builders, gave $600,000 to Priorities USA.

Tony and Heather Podesta, the Beltway lobbying power couple, gave $100,000 to Majority PAC, which supports Senate Democrats. The American Association for Justice, the trial lawyers’ trade association, whose annual conference was attended by Democratic super PAC officials, has given more than $500,000 to the groups.

Unexpectedly big givers for Democratic super PACs are building trade unions, which have combined to donate at least $6 million. The unions lobbied Obama aggressively to approve the full Keystone pipeline project connecting Canadian oil sands production with Gulf of Mexico refineries. They failed, at least temporarily, but the administration agreed to a southern section of the pipeline, guaranteeing thousands of jobs for their members.

"In many parts of the country in congressional and Senate races, we don’t have much member density, so it makes sense to use other tools, like super PACs," said Richard Greer, a spokesman for the Laborers’ International Union of North America.

The Democratic super PAC donor world remains far less robust than the Republican one. Total giving to their three groups reached $74 million through the beginning of September, not including donations to two affiliated tax-exempt groups that do not have to disclose their donors and fundraising to the Federal Election Commission.

That is far less than the $300 million that two groups co-founded by Republican strategist Karl Rove are aiming to raise.

But officials with the groups said they believed they had reached a tipping point, persuading skeptical Democrats — many with philosophical objections to super PACs — to embrace the new vehicles for unlimited spending.

"I think that as the threat of Republican money really materialized, Democrats really wanted to make sure that someone had the president’s back," said Bill Burton, a spokesman for Priorities USA. "I think it’s taken a longer time for Democrats to see the threat and impact of outside money."

Many of those who have given said they gave only reluctantly, despite opposing the new world of unlimited giving, out of a belief that supporting Democrats was the best way to change the rules.

Two new donors are Donald and Shelley Rubin, entrepreneurs who built one of the country’s largest health care companies and are better known for their philanthropic work, including founding a New York art museum. The couple gave $1 million in September.

"Donald and I believe strongly in the need for meaningful campaign finance reform and were deeply disappointed in the Supreme Court’s decision in Citizens United," Rubin said in a statement. "We also believe we cannot sit on the sidelines and simply let the president’s opponents capitalize on the court’s decision by bombarding the public with misleading and false attack ads."