The most significant players in the advertising assault on Senate Democratic candidates are the affiliated American Crossroads and Crossroads GPS, launched under the direction of former Bush administration political operative Karl Rove, and the U.S. Chamber of Commerce. Together, the three groups account for about $13 million in ad spending.
So far, the action is more limited in House races but still very much tilted toward Republicans.
Four GOP-leaning groups are spending more than $6 million combined in roughly three dozen races to less than $1 million for Democratic interests, led by the Service Employees International Union.
In many cases, the Republican allies are the only outside groups on the air. For example, Americans for Tax Reform is spending nearly $200,000 to help topple Democratic Rep. Ben Chandler of Kentucky, while Americans for Job Security is up with $230,000 in ads in New York's 24th Congressional District against Democratic Rep. Michael Arcuri.
Still, there are signs that Democratic-aligned groups are becoming more active in the final weeks. The SEIU has weighed in the last few days with $450,000 in Ohio's 16th Congressional District to help Democratic Rep. John Boccieri and $250,000 in Michigan's 7th district to counter attacks on freshman Democratic Rep. Mark Schauer.
The GOP is getting additional help from some groups that don't even weigh in directly in congressional races. Americans for Prosperity, a conservative group started by billionaire conservative David Koch, has spent about $5.5 million in key House battlegrounds with ads that don't mention candidates but criticize Obama's policies.
The financial advantage by outside GOP groups makes up for anemic fundraising by the Republican Party. The party's national committees trail their Democratic counterparts in overall contributions and overall cash on hand. That imbalance has helped Democrats respond to the avalanche of ads from GOP-allied groups in some key battlegrounds, but that is money Democrats also would have preferred to have in hand for a final campaign thrust in October.
The GOP groups appear to be benefiting from a Supreme Court ruling earlier this year that freed big business — which typically leans Republican — to spend their millions directly to sway federal elections. What's more, several of the groups are set up as nonprofit corporations under the Internal Revenue Service code, and they don't have to reveal their donors.
"You've gone to a world where the Supreme Court has said corporations have a constitutional right to do this spending," said Trevor Potter, a campaign finance lawyer who was counsel to John McCain's Republican presidential campaign. "That green light has been very important."