BOSTON — Mayors from across the nation on Sunday endorsed a campaign to uphold transgender protections during a gathering in Boston focused on issues impacting cities.

Over 300 officials attending the four-day U.S. Conference of Mayors meeting called on Massachusetts voters to uphold the state's transgender nondiscrimination law at the voting booth in November. They're dubbing their new coalition the bipartisan Mayors for Freedom Coalition of the Freedom for All Massachusetts campaign.

Boston Mayor Martin J. Walsh was one of over a dozen local public officials to sign on.

In 2016, the Massachusetts legislature passed nondiscrimination protections for transgender people in public places in the state. Shortly after it went into effect in October 2016, a group of opponents called Keep MA Safe gathered more than 34,200 signatures required to place the law on the ballot for repeal in November 2018. More than 32,375 were needed to ensure ballot placement.

"This law has been in place in Massachusetts for two years with no issues, and a similar local ordinance has been in place in Boston for more than a decade," Walsh said Sunday. "In that time, we have become a more welcoming and inclusive city for our transgender friends and neighbors."

Massachusetts is the first state in the nation where transgender protections will be up for a vote on a statewide ballot measure. Currently, Massachusetts is among 19 states and more than 250 municipalities with nondiscrimination protections for transgender people in public places, such as restaurants, shops, and medical offices. The Massachusetts law also protects transgender people from discrimination in public restrooms and locker rooms.

New Hampshire became the 19th state on Friday when Gov. Chris Sununu, a Republican, signed a law into effect.

The 86th annual meeting of the U.S. Conference of Mayors ends on Monday.