In a letter to members of Congress, Assistant Attorney General Peter Kadzik announced this afternoon that the Justice Department was ending its investigation of Lois Lerner, the former head of Exempt Organizations at the Internal Revenue Service. Lerner will not be prosecuted.

Lerner had been accused of singling out conservative groups for unfair scrutiny. While Kadzik found evidence of mismanagement and poor judgement, he did not conclude that agency personnel acted on partisan motives. He stated:

[The probe found] substantial evidence of mismanagement, poor judgment and institutional inertia leading to the belief by many tax-exempt applicants that the IRS targeted them based on their political viewpoints. But poor management is not a crime. We found no evidence that that any IRS official acted based on political discriminatory, corrupt or other inappropriate motives that would support a criminal prosecution.

Now that this probe has concluded, the IRS should devise more workable rules for political activity by 501(c)(4) nonprofits. The current IRS standard has allowed tens of millions of dollars to have been spent on federal campaigns without any disclosure of donors, and a Treasury Department investigation found that the vagueness of this rule contributed to the controversy that led to the investigation.

Where to start? The Sunlight Foundation supports the work of The Bright Lines Project to devise clear, objective rules for political activity by social welfare organizations.