The assault weapons ban was stripped out of the base legislation but is set to be voted on as an amendment. It is strongly opposed by the National Rifle Association and expected to fail.
Reid voted against renewing the 1994 assault weapons ban when it expired in 2004.
The majority leader signaled Tuesday afternoon that the spate of mass shootings in recent years had changed his views. On Wednesday morning, he said protecting police officers and innocent victims is "more important than preventing imaginary tyranny."
"The United States military is not out to get us. Federal law enforcement, local police departments are not out to get us," Reid said. "These conspiracy theories are dangerous and they should be put to rest."
He also came out for a ban on high-capacity ammunition magazines.
"My reason for wanting a ban on large capacity magazines in similar," he said. "These clips are designed to kill, not to kill a deer or a duck or any other animal large or small, they're designed to kill humans -- living, breathing human beings."