Missouri senators endorsed legislation on Tuesday that seeks to nullify U.S. gun restrictions and send federal agents to jail for enforcing such laws, though the measure would likely face a court challenge if it gets approved in the state.

Courts have consistently ruled that states cannot nullify federal laws, but that hasn't stopped Missouri and other states from trying.

Sen. Brian Nieves, the Republican sponsoring the bill, said the legislation would protect law-abiding gun owners from federal encroachments and regulations. Missouri Republicans began pushing for the legislation following President Obama's call last year for increased background checks and a ban on assault weapons.

The legislation would subject federal agents to civil and criminal penalties for knowingly enforcing federal gun laws. Agents could face up to one year in prison and a $1,000 fine.

"This is primarily purposed to protect liberties of Missourians," said Nieves, of Washington.

The measure also would give school districts the option to designate personnel to carry a concealed weapon in school buildings after undergoing training. Opponents argue that provision would increase access to firearms, which could then lead to more instances of gun violence.

"I cannot support this legislation in good conscience," said Sen. Jamilah Nasheed, D-St. Louis. "Kids are killing kids in school."

An amendment was added to require school districts to hold a public meeting before allowing personnel to carry in buildings. That provision's sponsor said he hopes that would cause districts to think twice before letting guns in the classroom.

"I believe my parents would say we don't want our teachers to carry guns and our school board members would then have to look at the consequences of that decision," said Rep. Jason Holsman, D-Kansas City.

Holsman's amendment would also allow designated personal to choose to carry pepper spray instead of a weapon.

Another provision of the bill would let holders of concealed gun permits carry firearms openly, even in municipalities with ordinances banning open carry. It would also lower the minimum age to get a concealed weapons permit to 19, down from 21. Under the bill, health care professionals could not be required to ask or document whether a patient owns a firearm.

The Senate also approved an amendment sponsored by Nasheed that would give gun owners 72 hours to report a stolen firearm to law enforcement.

Democratic Gov. Jay Nixon vetoed a similar measure that the Republican-controlled Legislature passed last year.

Nieves' bill is less specific than last year's version about which federal laws it seeks to nullify. It removes references to the 1934 and 1968 gun control acts, while keeping generic references to fees, registration and tracking policies that are considered "infringements" or "have a chilling effect on the purchase or ownership" of guns and ammunition by law-abiding citizens.

The measure needs one more affirmative vote in the Senate before heading to the House.