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 Grassroots Policy Strategies] 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.

   Neither the Internal Revenue Service, which has jurisdiction over nonprofits, nor the Federal Election Commission, which regulates the financing of federal races, appears likely to examine them closely, according to campaign finance watchdogs, lawyers who specialize in the field and current and former federal officials.

   A revamped regulatory landscape this year has elevated the attractiveness to political operatives of groups like Crossroads and others, organized under the auspices of Section 501(c) of the tax code. Unlike so-called 527 political organizations, which can also accept donations of unlimited size, 501(c) groups have the advantage of usually not having to disclose their donors’ identity.

   This is arguably more important than ever after the Supreme Court decision in the Citizens United case earlier this year that eased restrictions on corporate spending on campaigns.