Updated

Missouri Gov. Jay Nixon vetoed legislation Monday that would have allowed specially trained teachers to carry concealed guns, asserting that the move could jeopardize student safety in public schools.

The veto by the Democratic governor sets up a potential showdown with the Republican-led Legislature, which could override Nixon if it gets a two-thirds vote of both chambers during a September session.

Nixon announced the veto with a written statement on the deadline day for him to take action on bills passed earlier this year.

"Arming teachers will not make our schools safer," he said. "I have supported and will continue to support the use of duly authorized law enforcement officers employed as school resource officers, but I cannot condone putting firearms in the hands of educators who should be focused on teaching our kids."

The Missouri legislation called for allowing public school districts to designate certain teachers or administrators as "school protection officers," who would undergo special training to carry concealed weapons.

Supporters contend that armed school personnel could save students' lives by responding to an attacker without waiting precious minutes for police to arrive.

"I am disappointed this governor, who was all but absent during the process, has chosen to veto a bill designed to protect our children," said sponsor Will Kraus, R-Lee's Summit.

The Legislature began considering the measure after the deadly 2012 shooting at Sandy Hook Elementary School in Newtown, Connecticut. At least nine states passed bills last year authorizing armed school personnel and more than a dozen introduced similar measures this year.

The Missouri legislation also lowered the minimum age required to get a concealed weapons permit to 19 from 21 and allows permit holders to carry guns openly, even in cities that ban open carry. In addition, health care professionals could not be required to ask whether a patient has access to guns, and public housing authorities could not ban tenants from possessing firearms.

The bill passed the Missouri House in May by a 111-28 vote, two more than would be required for a veto override. The Senate's 21-7 vote fell two votes shy of that threshold, but three Republicans were absent.

With another veto Monday of legislation that would have barred minors from buying electronic cigarettes while also restricting further regulation, Nixon has rejected 33 bills approved by lawmakers this year, the most in one year since he took office in 2009 and among the most ever by a Missouri governor in a 12-month span.