Nashville's police union sued Tuesday to block a citywide vote on a community oversight board that would scrutinize law enforcement, a measure that was recently cleared for the November ballot.

In the suit, which followed an earlier threat of legal action, the Nashville Fraternal Order of Police argued the Davidson County Election Commission incorrectly approved the measure.

The FOP said Community Oversight Now, a group that pushed the referendum as a necessary way to root out racial bias and misconduct in the police force, had not met the signature threshold needed to get a measure added to the ballot.

The union asked the Davidson County Circuit Court to take quick action to stop the matter from going before voters in the Nov. 6 general election.

Community Oversight Now blasted the FOP lawsuit in a statement Tuesday, saying it "seeks to prevent Davidson County residents from voting on the most significant effort in decades to bring accountability to Metro Nashville government."

"The lawsuit is not just an attack on government accountability, but the sacred right to vote, which many sacrificed for the right to exercise," the group said.

The advocacy group urged Mayor David Briley and the Metro Council to oppose the lawsuit.

Dense government rules at the root of lawsuit

The lawsuit centers on language in the Metro Charter saying an amendment may be proposed to Davidson County voters if a petition is signed by 10 percent of the number of voters who voted in the "preceding general election."

Metro attorneys and Community Oversight Now have pointed to the August 2016 election, when 47,074 people voted. That made the threshold 4,708.

In all, Community Oversight Now had submitted more than 8,200 signatures. Election staff verified 4,801 and discarded around 1,200.

But the Nashville FOP, represented by prominent defense attorney David Raybin and Austin McMullen, argued the preceding general election actually occurred on May 24 when the city held a special mayoral election won by Briley. In that election, 82,368 people voted, meaning the bar would be higher, at 8,237 valid signatures.

The election commission rejected the FOP's argument last week and approved the measure for the ballot with a 5-0 vote. The union vowed to sue.

The oversight referendum would create an 11-member panel with the power to investigate allegations of police misconduct, including uses of force.

Police shootings sparked calls for oversight board

The effort to craft and push the referendum was galvanized by two shootings in which white police officers killed black men who were running from them. Community Oversight Now said those shootings, which killed Jocques Clemmons and Daniel Hambrick, were indicative of deep problems within the department that needed outside review.

In addition to reviewing police misconduct and recommending disciplinary actions, the board could suggest changes to police policies.

While the FOP's lawsuit focuses on procedural issues, the union remained vehemently opposed to the board itself. The suit said the board could upend officers' daily lives by "introducing uncertainty into their duties" and stripping their existing disciplinary system of due process.

"Our concern that the current proposed legislation lacks perspective from law enforcement, creates an environment lacking due process, and violates employee rights are the foundations of our position," FOP President James Smallwood said in a statement.

More:Daniel Hambrick shooting sparks strong reaction from community

From 2017:Nashville Fraternal Order of Police leader 'disturbed' by council effort to create oversight board

The union urged the court to decide the matter quickly, given that the election is less than three months away.

If the courts deny the FOP's request and the matter goes to voters in November, the FOP pledged to mount a "campaign to inform the voters."

Joey Garrison contributed to this report.

Reach Adam Tamburin at 615-726-5986 and atamburin@tennessean.com. Follow him on Twitter @tamburintweets.