A longtime political adviser to Mr. Soros, Michael Vachon, announced the donation at a luncheon Thursday hosted by the Democracy Alliance, said the two people, who were present. The alliance is a group of liberal donors who have already invested heavily in building grass-roots organizations and research institutes.
Mr. Soros, a retired hedge fund manager, will also give an additional $500,000 to two super PACs backing Congressional Democrats. Other donors at the lunch were expected to commit at least $10 million more to Democratic super PACs, suggesting that many — like Mr. Soros — had overcome their aversion to financing super PACs that focused only on advertising.
The gathering, which was headlined by former President Bill Clinton, suggested a rapprochement of sorts between progressive donors who have traditionally favored what they call “movement-building” and Democratic strategists who badly want large checks to pay for the party’s emerging super PAC apparatus, which has only recently begun to draw significant financial support, much of it from traditional party sources like Hollywood, trial lawyers and unions.
Senator Charles E. Schumer of New York and Representative Nancy Pelosi of California, the House Democratic leader, also attended the event, reflecting the growing embrace by party officials of super PACs supporting the party. The donors and officials were gathered at the Park Avenue apartment of Donald and Shelley Rubin, New York philanthropists and Obama donors who gave $1 million to Priorities USA this month.
“Donors are no longer sitting on the sidelines,” said Susan McCue, a spokeswoman for Majority PAC, which supports Senate Democrats. “They see how high the stakes are on key issues and with the Supreme Court.”
While Democratic super PACs have begun to attract a growing number of donors giving six- and seven-figure checks, they remain far behind their Republican equivalents in terms of fund-raising. The four top Democratic groups had together raised less through the beginning of September than Restore Our Future, which is backing Mitt Romney.
Mr. Soros’s own giving remains low compared with the biggest donors on the conservative side this cycle, like Sheldon Adelson, the casino owner, who has pledged to spend up to $100 million to defeat Mr. Obama and Democrats in Congress this year. Mr. Soros had previously given $1 million to American Bridge, a group that provides research, video and other services for its partner super PACs, and made low-six figure contributions to House Majority PAC, a group supporting House Democrats.
The biggest donor to Democratic super PACs, James Simons, who ran one of the world’s most successful hedge funds, contributed about $4 million through the beginning of September. About 20 individuals or couples have given the Democratic groups more than $1 million each, a category that is far larger on the Republican side, allowing groups like the Karl Rove-founded American Crossroads to raise and spend far more.
But Mr. Soros had suggested that he was unlikely to give to Priorities USA Action, and his shift may bring other donors into the fold. In 2004, he and a fellow billionaire, Peter B. Lewis, the chairman of Progressive Insurance, gave more than $40 million to Democratic-leaning outside groups, seed money for what became a $200 million effort.
The influx of big money now makes Democrats full participants in the system of unlimited spending that many in their party, including Mr. Obama, have vowed to change.
The late rush of Democratic donors “increases the threat of corruption by mega-donors, but also carries a silver lining from a reformer’s perspective,” said Paul S. Ryan, senior counsel at the Campaign Legal Center, which advocates for more restrictions on money in politics.
“With both Republicans and Democrats writing big checks to sway the election, and both parties’ candidates being beaten up by the outside groups’ ads, it will make it much harder for either party to dismiss the threat of these outside groups as overblown when it comes time to fix this broken system in 2013,” Mr. Ryan added.
Others who attended Thursday’s lunch included Tim Zagat, co-founder of the Zagat restaurant guide; Bernard L. Schwartz, a top Democratic donor; and Arnold Hiatt, the former president of Stride Rite who pledged $2 million to the Democrats’ efforts, according to people who attended.
Mr. Soros did not attend because the luncheon was held at the same time as a board meeting for the Open Society Foundations, which he founded.
But in an e-mail to other invitees, Mr. Soros sought to dispel suggestions that he was unhappy with Mr. Obama. “I fully support the re-election of President Obama,” Mr. Soros said.
He had not contributed until now, he wrote, because he opposed the Supreme Court’s Citizens United decision in 2010, which paved the way for super PACs and unlimited money in politics.
But, Mr. Soros wrote, he has become “appalled by the Romney campaign, which is openly soliciting the money of the rich to starve the state of the money it needs to provide social services.”