Generally, the spots are attack ads, but many feature cameos by the candidates themselves.
Donnelly's latest spot features him standing in the middle of a country road while an actor playing Mourdock yells at him: "Hey, Donnelly! It's my way or the highway!" The Mourdock camp hit back with its own country-road spot, featuring a car being driven by Donnelly taking a hard left at the end of the road before veering through the mud.
In the more rural Montana, where Democrats and Republicans each reserved roughly $3 million in airtime, the National Republican Senatorial Committee has attacked incumbent Democratic Sen. Jon Tester for opposing the repeal of federal inheritance taxes. Democrats and their allies have opted for a more populist attack against Republican candidate Denny Rehberg, attacking his comment last year about lobbying being "an honorable profession."
In addition to money from big outside groups like Crossroads and the U.S. Chamber of Commerce, buys are being made by lesser-known groups like the 60 Plus Association, a conservative alternative to the AARP, and Majority PAC, Senate Democrats' super PAC.
Democratic super PACs have been outspent by Republican-aligned groups roughly 2-to-1. Democratic Senatorial Campaign Committee spokesman Matt Canter argues his side has kept pace largely because Republican support groups are running outdated messages they used to corral tea party voters in 2010.
"Republicans continue to use the 2010 playbook that elected this tea party Congress and voters are sick and tired of the false partisan attacks," he said. Canter cites $20 million spent by Republican groups in Ohio and $17 million in Florida, then points to polls showing incumbent Democratic senators poised for re-election in those states.
But it's in states like Indiana — where outside spending has already well surpassed the $5.6 million spent in Indiana's 2010 Senate race that Republican Dan Coats won against former Rep. Brad Ellsworth — that the wash of money has been most surprising.
For example, Crossroads latest $4 million investment not only targets Florida and Virginia, but North Dakota and Montana.
Jason Miller, executive vice president of Jamestown Associates, which is writing ads for four Senate campaigns, including Mourdock's and Republican Linda Lingle's in Hawaii, said outside spending has made it more important for candidates to stick to their message.
"At the end of the day you still have to worry about playing your own game," he said.
Associated Press writer Kevin Freking contributed to this report from Washington.
- Wyoming ranked third in US in percentage of...
- Test anxiety: How cold feet are ruining your...
- Hometown pastor says community stands by...
- Sen. Harry Reid's retirement recalls his...
- 'A marvellous work and a wonder': A look back...
- Drownings along Rio Grande spike after...
- O'Malley: Presidency 'not some crown'...
- Hundreds rally against Indiana law, say it's...
- 'A marvellous work and a wonder': A... 64
- 'Maternal feminists' defend motherhood... 57
- Rep. Trey Gowdy: Hillary Clinton wiped... 49
- Sen. Harry Reid's retirement recalls... 39
- Senate's Harry Reid announces he won't... 34
- Alone at controls, Germanwings co-pilot... 27
- Hundreds rally against Indiana law, say... 22
- Delete, erase, shred: The enemies of... 15