Mayor Kirk Caldwell has allowed a controversial bill mandating unionized labor for some high-value city construction projects to become law without his signature despite legal concerns.

Caldwell said in a statement that he would like to see the new ordinance amended before it takes effect May 30, but it’s unclear whether that might happen.

The Honolulu City Council passed Bill 37 with a 7-2 vote Oct. 9.

At that time, Gary Kurokawa, chief of staff for Caldwell, said language in the bill might conflict with state procurement laws and that city attorneys were assessing the situation.

The bill would require something called a community workforce agreement, or CWA, that requires unionized labor and prohibits strikes for “critical” or “important” city construction contracts worth at least $2 million, including new buildings and infrastructure.

Councilman Joey Manahan, who introduced the bill, previously said the bill would apply to fewer than 20% of all city public works projects because projects without a critical need to meet construction deadlines would be excluded, such as redevelopment of the Neal Blaisdell Center and improvements to the Honolulu Zoo.

Caldwell, in a Thursday letter to the City Council, said the authority to decide whether a CWA should be part of a city construction project rests with the city’s chief procurement officer under state procurement code.

“The City Council lacks the authority to mandate such a requirement as a precondition for award of a contract,” Caldwell said in his letter.

The mayor also told the Council that a provision in Bill 37 that requires a preference be given to local laborers could jeopardize city projects using federal funding that precludes such a preference.

“As I have consistently indicated, I support the usefulness of project labor agreements such as a community workforce agreement,” Caldwell said in the letter. “I wholeheartedly agree that in various large public works projects, a CWA can promote efficient completion of construction projects by avoiding delays due to disruptions caused by labor strikes or slowdowns. Unfortunately, the mandatory nature of Bill 37 goes beyond the enactment of enabling legislation and infringes upon the authority of the executive branch to conduct procurements and negotiate an appropriate CWA to the benefit of the city.”

Caldwell noted in the letter that his understanding is that proponents of Bill 37 intend to ask state lawmakers to amend the Hawaii Public Procurement Code to address the authority issue.

The bill was initiated by the state’s most politically influential construction labor organizations — the Hawaii Construction Alliance and the Hawaii Building and Trades Council — and initially was drafted to apply to contracts valued at $250,000 or more.

Nonunionized contractors and local contractor organizations including the General Contractors Association of Hawaii and the Associated Builders & Contractors opposed the bill.