Manchin and Sen. Pat Toomey, R-Pa., reached a compromise on expanded backgrounds checks, a centerpiece of a push to limit gun sales in the wake of the Newtown, Conn., tragedy in which 26 people, 20 of them children, were killed in an elementary school in December.
The Manchin-Toomey amendment enjoyed majority support but couldn't clear the 60-vote threshold to avoid a filibuster.
Manchin told CBS's "This Morning" he was working to get more support by changing language he said opponents mischaracterized.
"We need to educate law-abiding gun owners, like myself, people that might belong to the NRA ... This bill not only protects your Second Amendment but expands the Second Amendment," Manchin said.
He noted the law protects sales or transfers between family members from performing a background check -- and that the bill would not lead to further gun regulations by the federal government without another act of Congress.