U.S. Sen. Tammy Baldwin, D-Wis., joined 40 Senate colleagues, led by Sen. Chris Murphy, D-Conn., to reintroduce the Background Check Expansion Act.

The bill would expand federal background checks to all gun sales. Under current federal law, unlicensed or private sellers are not required to conduct a background check prior to transferring a firearm.

About 90 percent of Americans support comprehensive background checks, according to the Washington Post. Research indicates that as many as a quarter of all gun sales in the United States may occur without a background check.

"If all Washington does is agree to disagree, then we have failed to provide the leadership people are demanding to reduce gun violence," Baldwin said in a prepared statement. "This legislation is a commonsense solution to prevent the easy access of weapons for those that shouldn't have them. We have a moral responsibility in Congress to step up, work together and address gun violence like the public health crisis that it is so we can start saving lives."

The Background Check Expansion Act will require background checks for the sale or transfer of all firearms. This requirement extends to all unlicensed sellers, whether they do business online, at gun shows or out of their home.

Exceptions to the Background Check Expansion Act include transfers between law enforcement officers; temporarily loaning firearms for hunting and sporting events; providing firearms as gifts to immediate family members; transferring a firearm as part of an inheritance; or temporarily transferring a firearm for immediate self-defense.

Democratic Sens. Tina Smith and Amy Klobuchar are among the 40 Democratic and Independent co-sponsors of the bill.