Crossroads Grassroots Policy Strategies would certainly seem to the casual observer to be a political organization: Karl Rove, a political adviser to President George W. Bush, helped raise money for it; the group is run by a cadre of experienced political hands; it has spent millions of dollars on television commercials attacking Democrats in key Senate races across the country.

Yet the Republican operatives who created the group earlier this year set it up as a 501(c)(4) nonprofit corporation, so its primary purpose, by law, is not supposed to be political.

The rule of thumb, in fact, is that more than 50 percent of a 501(c)(4)’s activities cannot be political. But that has not stopped Crossroads and a raft of other nonprofit advocacy groups like it  mostly on the Republican side, so far  from becoming some of the biggest players in this year’s midterm elections, in part because of the anonymity they afford donors, prompting outcries from campaign finance watchdogs.

The chances, however, that the flotilla of groups will draw much legal scrutiny for their campaign activities seem slim, because the organizations, which have been growing in popularity as conduits for large, unrestricted donations among both Republicans and Democrats since the 2006 election, fall into something of a regulatory netherworld.