"Conservative outside groups spent nearly a year airing ads in Michigan attacking Democratic Sen. Gary Peters and boosting his main Republican rival, John James.
The attacks went virtually unanswered. Then the cavalry arrived.
A trio of so-called super political action committees, or super PACs, tied to major national environmental groups launched a $1 million ad buy in April touting Peters' green record.
It was a sign of what would unfold in the following months as environmentalists became some of the biggest spenders in U.S. politics."