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

But Mr. Simons has given at least $2 million to Priorities USA Action, the “super PAC” aiding Mr. Obama, and $2 million more 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 had given at least $250,000 to the leading Democratic super PACs through the beginning of September, according to a New York Times analysis of campaign finance records, and dozens more have given $100,000 or more.