Imagine if liberal groups discovered that President Trump’s Internal Revenue Service was targeting them for heightened scrutiny or harassment. The media and Democrats would decry this assault on the First Amendment and declare the U.S. on the brink of autocracy. The scandal would dominate the midterms, and the legitimacy of the election would be called into question.

Strangely enough, the IRS did target organs of the opposition party during the last administration, but the episode has largely faded from public memory without resolution. May 10 marks the fifth anniversary of the revelation that President Obama’s IRS targeted conservative groups for more than two years prior to the 2012 presidential election.

While some of the faces at the IRS have changed, the law that enabled their misuse of power has not. Congress’s failure to address the problem leaves the U.S. democratic process vulnerable to further abuses.

Lois Lerner, the career official at the center of the IRS scandal, retired on full pension after invoking her Fifth Amendment right against self-incrimination before Congress. John Koskinen, appointed IRS commissioner by Mr. Obama to lead the agency “in difficult times,” served his full term, spending the better part of four years stonewalling congressional requests for information. On his watch, the IRS destroyed evidence subject to subpoena.

The response from the political system showed early promise but quickly fizzled. After initially expressing shock, Mr. Obama abandoned any pretext of interest, suggesting it was a “phony scandal.” And why not? A 2012 American Enterprise Institute study found that tea-party organizations substantially increased conservative turnout in the 2010 midterms. The agency’s suppression of those groups in the following years might have given Mr. Obama’s re-election a boost.