Democratic Senate candidate Jason Kander defends his gun control policies while assembling an AR-15 blindfolded in a new ad posted Thursday.

Mr. Kander, who is challenging Republican Roy Blunt for his Missouri Senate seat, talks about his service in the U.S. military and the importance of respecting firearms while he assembles the rifle in under 30 seconds.

“Sen. Blunt has been attacking me on guns. Well, in the Army, I learned how to use and respect my rifle. In Afghanistan, I volunteered to be an extra gun in a convoy of unarmored SUVs,” the Missouri secretary of state says.

“And in the State Legislature, I supported Second Amendment rights,” he continued. “I also believe in background checks, so the terrorists can’t get their hands on one of these.”

After assembling the AR-15, Mr. Kander takes his blindfold off and says, “I approve this message, because I would like to see Sen. Blunt do this.”

The National Rifle Association released an ad last week warning voters that Mr. Kander would vote against their Second Amendment rights.

“Missouri’s next senator will decide control of the Supreme Court,” the ad says. “Jason Kander refused to defend your Second Amendment rights in Jefferson City. How could you trust him in Washington?”

Mr. Kander is trailing Mr. Blunt by 5 percentage points in the RealClearPolitcs average of recent polls, The Hill reported.