The groups, including the Chamber of Commerce, the American Action Network and Crossroads GPS, which is linked to the Republican strategist Karl Rove, have committed to spending well over $150 million this year. President Obama has railed against these groups as they have poured money into races in which once-secure Democrats are hanging by a thread.

But the attacks may have only helped build the groups’ fund-raising muscle. Crossroads GPS and a sister organization, American Crossroads, have received more than $100,000 in small donations through the Web, when they had expected most gifts to come in big checks. And the groups’ leaders have only grown more influential — far more influential than the Republican National Committee, led by Michael Steele. Evidently, the corporate donors love having a secret route to influence politics and elect Republicans without showing their hands to a Washington still controlled by the Democrats.

In past elections, the Democrats have also used outside groups, including those organized by the party strategist Harold Ickes. In 2004, groups linked to the Democratic Party spent $150 million to influence the elections and agreed to pay $1.3 million in fines to settle charges that they had made illegal expenditures. In the last three elections, Democratic groups substantially outspent Republican groups. But many of these groups were so-called 527s, which were required to disclose donors’ identities.

In this election, Mr. Obama and the Democrats have either refused, or have been unable, to fight fire with similar, Democrat-leaning groups. With an angry Wall Street and donors like George Soros on the sidelines — “I don’t believe in standing in the way of an avalanche,” he recently said — the Democrats don’t have an obvious counter, except for labor unions, which probably can’t match corporate contributors.

Since Watergate, the names of political donors have largely been disclosed, even by so-called independent groups. In 2004 and 2006, nearly all independent groups involved in politics revealed their donors, according to a report by Public Citizen, a group that has long supported campaign finance reform. In 2008, fewer than half of these groups disclosed donors, and so far this year, fewer than one-third.

Because United States tax law permits certain social welfare and labor groups to collect donations anonymously if political activity is not their key focus, the only way to stop the undisclosed donations is to change the law. But Democrats recently failed to move a bill requiring disclosure through the Senate; not a single Republican voted for it.