The Senate is considering the Safe Communities, Safe Schools Act, S.649, which would expand background checks on gun purchases, crack down on gun trafficking and beef up security in schools. 

Some GOP senators have said the bill goes too far and infringes on the rights of gun-owners.

Democrats have been pushing for stricter gun laws since December, when a gunman killed 20 first-graders and six school employees at Sandy Hook Elementary School in Newtown, Conn.

“Losing a child is unnatural,” Murphy said. “But what is just as unnatural is a senator's unwillingness to do something about it. … These parents can’t understand the casual willingness of this chamber to turn our backs on those children.”

Manchin and Toomey’s amendment would expand background checks to cover all sales at gun shows and over the Internet. It would exempt gun sales and transfers between friends and acquaintances, and explicitly ban the federal government from creating a national firearms registry.

Murphy said that although the Manchin-Toomey amendment would not have stopped the tragedy in Newtown since the shooter’s mother bought the guns legally, the amendment would potentially stop criminals or those with mental illness from buying guns online or at a gun show.

The Senate is scheduled to vote on nine amendments Wednesday, all of which were subject to a 60-vote threshold. It is unclear if the Manchin-Toomey amendment can get 60 votes — it has the support of at least four Republicans, but not all 55 Senate Democrats are expected to support the measure.