The biggest independent spender is the conservative organization Americans for Prosperity.
A campaign-finance watchdog says independent groups are still dominating television advertising in Michigan's top two statewide political races.
Gubernatorial and U.S. Senate candidates accounted for about $4 million of about $18 million in ad spending through Monday.
The biggest independent spender is the conservative organization Americans for Prosperity, which the Michigan Campaign Finance Network says spent at least $4.3 million attacking Democratic Senate candidate Gary Peters.
Senate Democrats' super PAC spent $2 million criticizing Republican Senate candidate Terri Lynn Land.
In the governor's race, Republican Gov. Rick Snyder spent $1 million on ads and was aided by $1.5 million spent by the Republican Governors Association. Democrat Mark Schauer bought no ads and relied on $2.9 million in Democratic Governors Association TV ads.