The problem of big money in political campaigns isn’t just that there’s too much but that too much of it, with great potential for skewing election results, is being hidden from public view.

“Nonprofit” interest groups on the political right and left have taken advantage of a part of the federal tax code that was meant to help do-good organizations aimed at improving life in a community.

There’s a far cry between civic associations, sports league sponsors, volunteer fire companies and festival organizers — the examples the Internal Revenue Service cites as typically covered by 501(c)(4) of the tax code — and politically oriented groups that are raising, spending and donating megabucks to get their preferred candidates into federal office.

If groups like the Karl Rove-founded Crossroads GPS and the billionaire Koch brothers’ Americans for Prosperity want to butt heads with newer organizations like the pro-Obama Priorities USA, why should any of them receive benefits from U.S. taxpayers? And if they’re running multimillion-dollar operations while not paying taxes, why shouldn’t they have to disclose their money sources?

For the past year, the Campaign Legal Center and Democracy 21, which advocate campaign finance reform, have been asking the IRS to change antiquated rules that let big-money political groups exploit the tax code to operate like political action committees without the same openness requirements.

In a nutshell, the groups set up as 501(c)(4) nonprofits ostensibly dedicated to “social welfare,” they collect gobs of money from donors whose identities don’t have to be made public, and then they disburse the funds to their affiliated super PACs, candidates and like-minded entities or do their own advocacy. And regular voters get to know very little about who’s behind it all despite its disproportionate influence.

The IRS advises that to qualify as tax-exempt under 501(c)(4), a group “must be operated exclusively to promote social welfare.” Lobbying for legislation is acceptable; so are “issue ads” aimed largely at public education.

Ads that name candidates, either for or against, aren’t allowed. But a social welfare organization “may engage in some political activities, so long as that is not its primary activity.”

Because the IRS hasn’t defined “primary activity,” crafty politicos apparently are operating on the rationale that they can spend up to 49 percent of their revenue on political activities without that counting as their “primary activity.”

Sure, that sounds preposterous.

Late-night cable TV comedian Stephen Colbert underscored the absurdity by creating his own 501(c)(4), called “Anonymous Shell Corporation,” to funnel unlimited anonymous donations to his super PAC (also known as “Americans for a Better Tomorrow, Tomorrow”), another entity designed to mock current campaign finance laxity.

Last week, the IRS notified Democracy 21 and the Campaign Legal Center that it would consider changing the rules for “social welfare” nonprofits, regulations that date to 1959.

It’s about time enforcement of the tax code caught up to modern politics. It’s unfortunate that, given the laboriousness of any federal rule-making, revisions almost certainly won’t help for this year’s elections.

The Center for Public Integrity (www.iwatchnews.org) and the Center for Responsive Politics (www.opensecrets.org) reported recently that more than 100 nonprofits organized as tax-exempt 501(c)(4) groups spent about $95 million on political activity in the 2010 elections — more than the $65 million spent by super PACs.

The report also said that conservative “social welfare” groups spent $78 million to liberal groups’ $16 million.

What’s important is not which side is more adept at fundraising; it’s whether a valuable public benefit is being manipulated. Tax-exempt status is intended to promote the public good, not enable wealthy and powerful donors of any political stripe to wield influence without public scrutiny.