In November, when the Internal Revenue Service finally stirred itself to propose a modest crackdown on the abuse of the tax code by political groups, it was immediately attacked by tax-exempt nonprofit groups on the right. That wasn’t too surprising; secret donations from conservatives to these groups are the principal reason American politics is now dominated by those with huge bank accounts.

But now liberal tax-exempt groups are also raising their voice in protest over the I.R.S.’s plans, afraid that they will be caught in the same crackdown, and will be unable to engage in political activity. The best thing the I.R.S. can do is to ignore both sides and proceed swiftly ahead, making its proposed rules even stronger to squeeze the influence of money out of politics.

The problem of secret money began in 2010, with the loosening of rules that was prompted in part by the Supreme Court’s Citizens United decision. Political operatives like Karl Rove realized that “social welfare” groups were allowed by the tax code to accept unlimited donations that did not have to be disclosed. They could then use that money to run political attack ads. Though the tax code says the groups, known as 501(c)(4)s, could not be engaged primarily in political activity and still keep their tax exemption, that was easy enough to get around by claiming the ads had some kind of civic purpose.

By the 2012 election, these groups were spending $300 million and were often the dominant voice in major races. The Koch brothers, in particular, got around the tax code provision by moving tens of millions among a huge number of nonprofits so that it was almost impossible to determine the purpose of each group, let alone who the donors were.